,illustration3 ,,plate ,i ,frontispiece ,,gem-,,stones ,,gem-,,stones ,,and ,,their ,,distinctive ,,characters ,,by ,g4 ,f4 ,,herbert ,,smith ,m4,a41 ,d4,sc4 ,,of ,,the ,,british ,,museum 7,,natural ,,history7 ,,with ,,many ,,diagrams ,,and ,,thirty-,,two ,,plates ,,of ,,which ,,three ,,are ,,in ,,colour ,,third ,,edition ,,methuen ,,co4 ,,ltd4 #cf ,,essex ,,street ,w4,c4 ,,london ,first ,published ,march #bast #aiaj ,second ,edition ,june #aiac ,third ,edition #aiai ,,preface ,in this edition the opportunity has been taken to correct a few misprints and mistakes that have been discovered in the first1 and to alter slightly one or two paragraphs1 but otherwise no change has been made4 ,g4 ,f4 ,h4 ,s4 ,,wandsworth ,,common1 ,s4,w4 ,,preface ,,to ,,the ,,first ,,edition ,it has been my endeavour to provide in this book a concise1 yet sufficiently complete1 account of the physical characters of the mineral species which find service in jewellery1 and of the methods available for determining their principal physical constants to enable a reader1 even if previously unacquainted with the subject1 to have at hand all the information requisite for the sure identification of any cut stone which may be met with4 ,for several reasons ,i have dealt somewhat more fully with the branches of science closely connected with the properties of crystallized matter than has been customary hitherto in even the most comprehensive books on precious stones4 ,recent years have witnessed many changes in the jewellery world4 ,gem-stones are no longer entirely drawn from a few well-marked mineral species1 which are1 on the whole1 easily distinguishable from one another1 and it becomes increasingly difficult for even the most experienced eye to recognize a cut stone with unerring certainty4 ,so long as the only confusion lay between precious stones and paste imitations an ordinary file was the solitary piece of apparatus required by the jeweller1 but now recourse must be had to more discriminative tests1 such as the refractive index or the specific gravity1 the determination of which calls for a little knowledge and skill4 ,concurrently1 a keener interest is being taken in the scientific aspect of gem-stones by the public at large1 who are attracted to them mainly by sthetic considerations4 ,while the treatment has been kept as simple as possible1 technical expressions1 where necessary1 have not been avoided1 but their meanings have been explained1 and it is hoped that their use will not prove stumbling-blocks to the novice4 ,unfamiliar words of this kind often give a forbidding air to a new subject1 but they are used merely to avoid circumlocution1 and not1 like the incantations of a wizard1 to veil the difficulties in still deeper gloom4 ,for actual practical work the pages on the refractometer and its use and the method of heavy liquids for the determination of specific gravities1 and the tables of physical constants at the end of the book1 with occasional reference1 in case of doubt1 to the descriptions of the several species alone are required2 other methods--such as the prismatic mode of measuring refractive indices1 or the hydrostatic way of finding specific gravities--which find a place in the ordinary curriculum of a physics course are described in their special application to gem-stones1 but they are not so suitable for workshop practice4 ,since the scope of the book is confined mainly to the stones as they appear on the market1 little has been said about their geological occurrence2 the case of diamond1 however1 is of exceptional interest and has been more fully treated4 ,the weights stated for the historical diamonds are those usually published1 and are probably in many instances far from correct1 but they serve to give an idea of the sizes of the stones2 the ,english carat is the unit used1 and the numbers must be increased by about #b per cent4 if the weights be expressed in metric carats4 ,the prices quoted for the various species must only be regarded as approximate1 since they may change from year to year1 or even day to day1 according to the state of trade and the whim of fashion4 ,the diagram on ,plate ,,ii and most of the crystal drawings were made by me4 ,the remaining drawings are the work of ,mr4 ,h4 ,h4 ,penton4 ,he likewise prepared the coloured drawings of cut stones which appear on the three coloured plates1 his models1 with two exceptions1 being selected from the cut specimens in the ,mineral ,collection of the ,british ,museum by permission of the ,trustees4 ,unfortunately1 the difficulties that still beset the reproduction of pictures in colour have prevented full justice being done to the faithfulness of his brush4 ,i highly appreciate the interest he took in the work1 and the care and skill with which it was executed4 ,my thanks are due to the ,de ,beers ,consolidated ,mines ,co4 ,ltd41 and to ,sir ,henry ,a4 ,miers1 ,f4,r4,s41 ,principal of the ,university of ,london1 for the illustrations of the ,kimberley and ,wesselton diamond mines1 and of the methods and apparatus employed in breaking up and concentrating the blue ground2 to ,messrs4 ,i4 ,j4 ,asscher ,co4 for the use of the photograph of the ,cullinan diamond2 to ,mr4 ,j4 ,h4 ,steward for the loan of the block of the refractometer2 and to ,mr4 ,h4 ,w4 ,atkinson for the illustration of the diamond-sorting machine4 ,my colleague1 ,mr4 ,w4 ,campbell ,smith1 ,b4,a41 has most kindly read the proof-sheets1 and has been of great assistance in many ways4 ,i hope that1 thanks to his invaluable help1 the errors in the book which may have escaped notice will prove few in number and unimportant in character4 ,to ,mr4 ,edward ,hopkins ,i owe an especial debt of gratitude for his cheerful readiness to assist me in any way in his power4 ,he read both the manuscript and the proof-sheets1 and the information with regard to the commercial and practical side of the subject was very largely supplied by him4 ,he also placed at my service a large number of photographs1 some of which--for instance1 those illustrating the cutting of stones--he had specially taken for me1 and he procured for me the jewellery designs shown on ,plates ,,iv and ,v4 ,if this book be found by those engaged in the jewellery trade helpful in their everyday work1 and if it wakens in readers generally an appreciation of the variety of beautiful minerals suitable for gems1 and an interest in the wondrous qualities of crystallized substances1 ,i shall be more than satisfied4 ,g4 ,f4 ,h4 ,s4 ,,wandsworth ,,common1 ,s4,w4 ,,contents ,,chap4 ,,page ,i4 ,,introduction #a ,,part ,i--,,section ,a ,,the ,,characters ,,of ,,gem-,,stones ,,ii4 ,,crystalline ,,form #f ,,iii4 ,,reflection1 ,,refraction1 ,,and ,,dispersion #ad ,,iv4 ,,measurement ,,of ,,refractive ,,indices #ba ,v4 ,,lustre ,,and ,,sheen #cg ,,vi4 ,,double ,,refraction #dj ,,vii4 ,,absorption ,,effects3 ,,colour1 ,,dichroism1,,etc4 #ec ,,viii4 ,,specific ,,gravity #fc ,,ix4 ,,hardness ,,and ,,cleavability #gh ,x4 ,,electrical ,,characters #hb ,,part ,i--,,section ,b ,,the ,,technology ,,of ,,gem-,,stones ,,xi4 ,,unit ,,of ,,weight #hd ,,xii4 ,,fashioning ,,of ,,gem-,,stones #hh ,,xiii4 ,,nomenclature ,,of ,,precious ,,stones #aji ,,xiv4 ,,manufactured ,,stones #aac ,,xv4 ,,imitation ,,stones #abd ,,part ,,ii--,,section ,a ,,precious ,,stones ,,xvi4 ,,diamond #abh ,,xvii4 ,,occurrence ,,of ,,diamond #acg ,,xviii4 ,,historical ,,diamonds #aeg ,,xix4 ,,corundum 7,sapphire1 ,ruby7 #agb ,,xx4 ,,beryl 7,emerald1 ,aquamarine1 ,morganite7 #ahd ,,part ,,ii--,,section ,b ,,semi-,,precious ,,stones ,,xxi4 ,,topaz #aig ,,xxii4 ,,spinel 7,balas-,ruby1 ,rubicelle7 #bjc ,,xxiii4 ,,garnet #bjg 7a7 ,,hessonite 7,grossular1 ,cinnamon-,stone1 ,hyacinth1 ,jacinth7 #baa 7b7 ,,pyrope 7#8,cape-,ruby'7 #bab 7c7 ,,rhodolite #bad 7d7 ,,almandine 7,carbuncle7 #bad 7e7 ,,spessartite #baf 7f7 ,,andradite 7,demantoid1 ,topazolite1 #8,olivine'7 #baf 7g7 ,,uvarovite #bah ,,xxiv4 ,,tourmaline 7,rubellite7 #bai ,,xxv4 ,,peridot #bbe ,,xxvi4 ,,zircon 7,jargoon1 ,hyacinth1 ,jacinth7 #bbh ,,xxvii4 ,,chrysoberyl 7,chrysolite1 ,cat's-,eye1 ,cymophane1 ,alexandrite7 #bcc ,,xxviii4 ,,quartz 7,rock-,crystal1 ,amethyst1 ,citrine1 ,cairngorm1 ,cat's-,eye1 ,tiger's-,eye7 #bch ,,xxix4 ,,chalcedony1 ,,agate1 ,,etc4 #bdf ,,xxx4 ,,opal 7,white ,opal1 ,black ,opal1 ,fire-,opal7 #bdi ,,xxxi4 ,,felspar 7,moonstone1 ,sunstone1 ,labradorite1 ,amazon-,stone7 #bed ,,xxxii4 ,,turquoise1 ,,odontolite1 ,,variscite #beg ,,xxxiii4 ,,jade 7,,nephrite ,,or ,,greenstone1 ,jadeite7 #bfj ,,xxxiv4 ,,spodumene 7,kunzite1 ,hiddenite71 ,,iolite1 ,,benitoite #bfe ,,xxxv4 ,,euclase1 ,,phenakite1 ,,beryllonite #bfi ,,xxxvi4 ,,enstatite 7#8,green ,garnet'71 ,,diopside1 ,,kyanite1 ,,andalusite1 ,,idocrase1 ,,epidote1 ,,sphene1 ,,axinite1 ,,prehnite1 ,,apatite1 ,,dioptase #bga ,,xxxvii4 ,,cassiterite1 ,,anatase1 ,,pyrites1 ,,hematite #bha ,,xxxviii4 ,,obsidian1 ,,moldavite #bhc ,,part ,,ii--,,section ,c ,,ornamental ,,stones ,,xxxix4 ,,fluor1 ,,lapis ,,lazuli1 ,,sodalite1 ,,violane1 ,,rhodonite1 ,,azurite1 ,,malachite1 ,,thulite1 ,,marble1 ,,apophyllite1 ,,chrysocolla1 ,,steatite ,,or ,,soapstone1 ,,meerschaum1 ,,serpentine #bhe ,,part ,,ii--,,section ,d ,,organic ,,products ,,xl4 ,,pearl1 ,,coral1 ,,amber #bia ,,tables ,i4 ,,chemical ,,composition ,,of ,,gem-,,stones #cjj ,,ii4 ,,colour ,,of ,,gem-,,stones #cja ,,iii4 ,,refractive ,,indices ,,of ,,gem-,,stones #cjb ,,iv4 ,,colour-,,dispersion ,,of ,,gem-,,stones #cjc ,v4 ,,character ,,of ,,the ,,refraction ,,of ,,gem-,,stones #cjc ,,vi4 ,,dichroism ,,of ,,gem-,,stones #cjd ,,vii4 ,,specific ,,gravities ,,of ,,gem-,,stones #cje ,,viii4 ,,degrees ,,of ,,hardness ,,of ,,gem-,,stones #cje ,,ix4 ,,data #cjf ,,index #cjg ,,list ,,of ,,plates ,,page ,i4 ,,gem-,,stones 7in colour7 ,frontispiece ,,ii4 ,,refractive ,,index ,,diagram #cf ,,iii4 ,,interference ,,figures #dh ,,iv4 ,,jewellery ,,designs #fb ,v4 ,,jewellery ,,designs #hh ,,vi4 ,,appliances ,,used ,,for ,,polishing ,,diamonds #ajb ,,vii4 ,,polishing ,,diamonds #ajc ,,viii4 ,,slitting ,,and ,,polishing ,,coloured ,,stones #ajd ,,ix4 ,,faceting ,,machine #aje ,x4 ,,lapidary',s ,,workshop ,,and ,,office ,,in ,,england #ajf ,,xi4 ,,lapidary',s ,,workshop ,,in ,,russia #ajg ,,xii4 ,,french ,,family ,,cutting ,,stones #ajh ,,xiii4 ,,indian ,,lapidary #aji ,,xiv4 ,,blowpipe ,,used ,,for ,,the ,,manufacture ,,of ,,rubies ,,and ,,sapphires #aah ,,xv4 ,,kimberley ,,mine1 #ahga #adj ,,xvi4 ,,kimberley ,,mine1 #ahgb #ada ,,xvii4 ,,kimberley ,,mine1 #ahgd #adb ,,xviii4 ,,kimberley ,,mine1 #ahha #adc ,,xix4 ,,kimberley ,,mine ,,at ,,the ,,present ,,day #add ,,xx4 ,,wesselton 7open7 ,,mine #ade ,,xxi4 ,,loading ,,the ,,blue ,,ground ,,on ,,the ,,floors1 ,,and ,,ploughing ,,it ,,over #adf ,,xxii4 ,,washing-,,machines ,,for ,,concentrating ,,the ,,blue ,,ground #adg ,,xxiii4 ,,diamond-,,sorting ,,machines #adh ,,xxiv4 ,,kafirs ,,picking ,,out ,,diamonds #adi ,,xxv4 ,,cullinan ,,diamond 7natural size7 #afh ,,xxvi4 ,,large ,,aquamarine ,,crystal 7one-sixth natural size71 ,,found ,,at ,,marambaya1 ,,minas ,,geraes1 ,,brazil #aif ,,xxvii4 ,,gem-,,stones 7in colour7 #bbf ,,xxviii4 ,,opal ,,mines1 ,,white ,,cliffs1 ,,new ,,south ,,wales #beb ,,xxix4 ,,gem-,,stones 7in colour7 #bef ,,xxx4 ,,natives ,,drilling ,,pearls #bid ,,xxxi4 ,,metal ,,figures ,,of ,,buddha ,,inserted ,,in ,a ,,pearl-,,oyster #bif ,,xxxii4 ,,sections ,,of ,,culture ,,pearl #big ,,gem-,,stones ,,chapter ,i ,,introduction ,beauty1 durability1 and rarity3 such are the three cardinal virtues of a perfect gem-stone4 ,stones lacking any of them cannot aspire to a high place in the ranks of precious stones1 although it does not necessarily follow that they are of no use for ornamental purposes4 ,the case of pearl1 which1 though not properly included among gem-stones1 being directly produced by living agency1 yet holds an honoured place in jewellery1 constitutes to some extent an exception1 since its incontestable beauty atones for its comparative want of durability4 ,that a gem-stone should be a delight to the eye is a truism that need not be laboured2 for such is its whole raison d'tre4 ,the members of the ,mineral ,kingdom that find service in jewellery may be divided into three groups1 according as they are transparent1 translucent1 or opaque4 ,of these the first1 which is by far the largest and the most important1 may itself be further sub-divided into two sections3 stones which are devoid of colour1 and stones which are tinted4 ,among the former1 diamond reigns supreme1 since it alone possesses that marvellous #8fire1' oscillating with every movement from heavenly blue to glowing red1 which is so highly esteemed and so much besought4 ,other stones1 such as #8fired' zircon1 white sapphire1 white topaz1 and rock-crystal1 may dazzle with brilliancy of light reflected from the surface or emitted from the interior1 but none of them1 like diamond1 glow with mysterious gleams4 ,no hint of colour1 save perhaps a trace of the blue of steel1 can be tolerated in stones of this category2 above all is a touch of the jaundice hue of yellow abhorred4 ,it taxes all the skill of the lapidary to assure that the disposition of the facets be such as to reveal the full splendour of the stone4 ,a coloured stone1 on the other hand1 depends for its attractiveness more upon its intrinsic hue than upon the manner of its cutting4 ,the tint must not be too light or too dark in shade3 a stone that has barely any colour has little interest1 and one which is too dark appears almost opaque and black4 ,the lapidary can to some extent remedy these defects by cutting the former deep and the latter shallow4 ,in certain curious stones--for instance tourmaline--the transparency1 and in others--such as ruby1 sapphire1 and one of the recent additions to the gem world1 kunzite--the colour1 varies considerably in different directions4 ,the colours that are most admired--the fiery red of ruby1 the royal blue of sapphire1 the verdant green of emerald1 and the golden yellow of topaz--are pure tints1 and the absorption spectra corresponding to them are on the whole continuous and often restricted4 ,they therefore retain the purity of their colour even in artificial light1 though certain sapphires transmit a relatively larger amount of red1 and consequently turn purple at night4 ,of the small group of translucent stones which pass light1 but are not clear enough to be seen through1 the most important is opal4 ,it and certain others of the group owe their merit to the same optical effect as that characterizing soap-bubbles1 tarnished steel1 and so forth1 and not to any intrinsic coloration4 ,another set of stones--moonstone and the star-stones--reflect light from the interior more or less regularly1 but not in such a way as to produce a play of colour4 ,the last group1 which comprises opaque stones1 has a single representative among ordinary gem-stones1 namely1 turquoise4 ,in this case light is scattered and reflected from layers immediately contiguous to the surface1 and the colour is due to the resulting absorption4 ,the apparent darkness of a deep-coloured stone follows from a different cause3 the light passing into the stone is wholly absorbed within it1 and1 since none is emitted1 the stone appears black4 ,the claims of turquoise are maintained by the blue variety2 there is little demand for stones of a greenish tinge4 ,it is evidently desirable that any stones used in jewellery should be able to resist the mechanical and chemical actions of everyday life4 ,no one is anxious to replace jewels every few years1 and the most valuable stones are expected to endure for all time4 ,the mechanical abrasion is caused by the minute grains of sand that are contained in ordinary dust1 and gem-stones should be at least as hard as they--a condition fulfilled by all the principal species with the exception of opal1 turquoise1 peridot1 and demantoid4 ,since the beauty of the first named does not depend on the brilliancy of its polish1 scratches on the surface are not of much importance2 further1 all four are only slightly softer than sand4 ,it may be noted that the softness of paste stones1 apart from any objections that may be felt to the use of imitations1 renders them unsuitable for jewellery purposes4 ,the only stones that are likely to be chemically affected in the course of wear are those which are in the slightest degree porous4 ,it is hazardous to immerse turquoises in liquids1 even in water1 lest the bluish green colour be oxidized to the despised yellowish hue4 ,the risk of damage to opals1 moonstones1 and star-stones by the penetration of dirt or grease into the interior of the stones is less1 but is not wholly negligible4 ,similar remarks apply with even greater force to pearls4 ,their charm1 which is due to a peculiar surface-play of light1 might be destroyed by contamination with grease1 ink1 or similar matter2 they are1 moreover1 soft4 ,for both reasons their use in rings is much to be deprecated4 ,nothing can be more unsightly than the dingy appearance of a pearl ring after a few years' wear4 ,it cannot be gainsaid that mankind prefers the rare to the beautiful1 and what is within reach of all is lightly esteemed4 ,it is for this reason that garnet and moonstone lie under a cloud4 ,purchasers can readily be found for a #8,cape-ruby' or an #8olivine1' but not for a garnet2 garnets are so common1 is the usual remark4 ,nevertheless1 the stones mentioned are really garnets4 ,if science succeeded in manufacturing diamonds at the cost of shillings instead of the pounds that are now asked for ,nature's products--not that such a prospect is at all probable or even feasible--we might expect them to vanish entirely from fashionable jewellery4 ,a careful study of the showcases of the most extensive jewellery establishment brings to light the fact that1 despite the apparent profusion1 the number of different species represented is restricted4 ,diamond1 ruby1 emerald1 sapphire1 pearl1 opal1 turquoise1 topaz1 amethyst are all that are ordinarily asked for4 ,yet1 as later pages will show1 there are many others worthy of consideration2 two among them--peridot and tourmaline--are1 indeed1 slowly becoming known4 ,for the first five of the stones mentioned above1 the demand is relatively steady1 and varies absolutely only with the purchasing power of the world2 but a lesser known stone may suddenly spring into prominence owing to the caprice of fashion or the preference of some great lady or leader of fashion4 ,not many years ago1 for instance1 violet was the favourite colour for ladies' dresses1 and consequently amethysts were much worn to match1 but with the change of fashion they speedily sank to their former obscurity4 ,another stone may perhaps figure at some royal wedding2 for a brief while it becomes the vogue1 and afterwards is seldom seen4 ,except that diamond1 ruby1 emerald1 and sapphire1 and1 we should add1 pearl1 may indisputably be considered to occupy the first rank1 it is impossible to form the gem-stones in any strict order4 ,every generation sees some change4 ,the value of a stone is after all merely what it will fetch in the open market1 and its artistic merits may be a matter of opinion4 ,the familiar aphorism1 de gustibus non est disputandum1 is a warning not to enlarge upon this point4 ,,part ,i--,,section ,a ,,the ,,characters ,,of ,,gem-,,stones ,,chapter ,,ii ,,crystalline ,,form ,with the single exception of opal1 the whole of the principal mineral species used in jewellery are distinguished from glass and similar substances by one fundamental difference3 they are crystallized matter1 and the atoms composing them are regularly arranged throughout the structure4 ,the words crystal and glass are employed in science in senses differing considerably from those in popular use4 ,the former of them is derived from the ,greek word 1 meaning ice1 and was at one time used in that sense4 ,for instance1 the old fourteenth-century reading of ,psalm cxlvii4 #ag1 which appears in the authorized version as 8,he giveth his ice like morsels10 ran 8,he sendis his kristall as morcels40 ,it was also applied to the beautiful1 lustrous quartz found among the eternal snows of the ,alps1 since1 on account of their limpidity1 these stones were supposed1 as ,pliny tells us1 to consist of water congealed by the extreme cold of those regions2 such at the present day is the ordinary meaning of the word4 ,but1 when early investigators discovered that a salt solution on evaporation left behind groups of slender glistening prisms1 each very similar to the rest1 they naturally--though1 as we now know1 wrongly--regarded them as representing yet another form of congealed water1 and applied the same word to such substances4 ,subsequent research has shown that these salts1 as well as mineral substances occurring with natural faces in nature1 have in common the fundamental property of regularity of arrangement of the constituent atoms1 and science therefore defines by the word crystal a substance in which the structure is uniform throughout1 and all the similar atoms composing it are arranged with regard to the structure in a similar way4 ,the other word is yet more familiar2 it denotes the transparent1 lustrous1 hard1 and brittle substance produced by the fusion of sand with soda or potash or both which fills our windows and serves a variety of useful purposes4 ,research has shown that glass1 though apparently so uniform in character1 has in reality no regularity of molecular arrangement4 ,it is1 in fact1 a kind of mosaic of atoms1 huddled together anyhow1 but so irregular is its irregularity that it simulates perfect regularity4 ,science uses the word glass in this widened meaning4 ,two substances may1 as a matter of fact1 have the same chemical composition1 and one be a crystal and the other a glass4 ,for example1 quartz1 if heated to a high temperature1 may be fused and converted into a glass4 ,the difference in the two types of structure may be illustrated by a comparison between a regiment of soldiers drawn up on parade and an ordinary crowd of people4 ,the crystalline form is a visible sign of the molecular arrangement1 and is intimately associated with the directional physical properties1 such as the optical characters1 cleavage1 etc4 ,a study of it is not only of interest in itself1 but also of great importance to the lapidary who wishes to cut a stone to the best advantage1 and it is1 moreover1 of service in distinguishing stones when in the rough state4 ,illustration3 ,,fig4 #a4--;,cubo-,octahedra4 ,the development of natural faces on a crystal is far from being haphazard1 but is governed by the condition that the angles between similar faces1 whether on the same crystal or on different crystals1 are equal1 however varying may be the shapes and the relative sizes of the faces 7,fig4 #a71 and by the tendency of the faces bounding the crystal to fall into series with parallel edges1 such series being termed zones4 ,each species has a characteristic type of crystallization1 which may be referred to one of the following six systems3-- #a4 ,cubic4--,crystals in this system can be referred to three edges1 which are mutually at right angles1 and in which the directional characters are identical in value4 ,these principal edges are known as axes4 ,some typical forms are the cube 7,fig4 #b71 characteristic of fluor2 the octahedron 7,fig4 #c71 characteristic of diamond and spinel2 the dodecahedron 7,fig4 #d71 characteristic of garnet2 and the triakisoctahedron1 or three-faced octahedron 7,fig4 #e74 ,illustration3 ,,fig4 #b4--;,cube4 ,illustration3 ,,fig4 #c4--,octahedron4 ,illustration3 ,,fig4 #d4--;,dodecahedron4 ,all crystals belonging to this system are singly refractive4 #b4 ,tetragonal4--,such crystals can be referred to three axes1 which are mutually at right angles1 but in only two of them are the directional characters identical4 ,a typical form is a four-sided prism1 mm1 of square section1 terminated by four triangular faces1 p 7,fig4 #f71 the usual shape of crystals of zircon and idocrase4 ,illustration3 ,,fig4 #e4--,triakisoctahedron1 or ,three-faced ,octahedron4 ,illustration3 ,,fig4 #f4--,tetragonal ,crystal4 ,crystals belonging to this system are doubly refractive and uniaxial1 i4e4 they have one direction of single refraction 7cf4 p4 #de71 which is parallel to the unequal edge of the three mentioned above4 ,illustration3 ,,fig4 #g4--,two alternative sets of ,axes in the ,hexagonal ,system4 #c4 ,hexagonal4--,such crystals can be referred alternatively either to a set of three axes1 ,x1 ,y1 ,z 7,fig4 #g71 which lie in a plane perpendicular to a fourth1 ,h1 and are mutually inclined at angles of #fj1 or to a set of three1 a1 b1 c1 which are not at right angles as in the cubic system1 but in which the directional characters are identical4 ,the fourth axis in the first arrangement is equally inclined to each in the second set of axes4 ,many important species crystallize in this system--corundum 7sapphire1 ruby71 beryl 7emerald1 aquamarine71 tourmaline1 quartz1 and phenakite4 ,the crystals usually display a six-sided prism1 terminated by a single face1 c1 perpendicular to the edge of the prism m 7,fig4 #h71 e4g4 emerald1 or by six or twelve inclined faces1 p 7,fig4 #i71 e4g4 quartz1 crystals of which are so constant in form as to be the most familiar in the ,mineral ,kingdom4 ,tourmaline crystals 7,fig4 #aj7 are peculiar because of the fact that often one end is obviously to the eye flatter than the other4 ,illustration3 ,,figs4 #h-aj4--;,hexagonal ,crystals4 ,crystals belonging to this system are also doubly refractive and uniaxial1 the direction of single refraction being parallel to the fourth axis mentioned above1 and therefore also parallel to the prism edge4 ,hence deeply coloured tourmaline1 which strongly absorbs the ordinary ray1 must be cut with the table-facet parallel to the edge of the prism4 ,illustration3 ,,fig4 #aa4--,relation of the two directions of single ,refraction to the ,axes in an ,orthorhombic ,crystal4 #d4 ,orthorhombic4--,such crystals can be referred to three axes1 which are mutually at right angles1 but in which each of the directional characters are different4 ,the crystals are usually prismatic in shape1 one of the axes being parallel to the prism edge4 ,topaz1 peridot1 and chrysoberyl are the most important species crystallizing in this system4 ,crystals belonging to this system are doubly refractive and biaxial1 i4e4 they have two directions of single refraction 7cf4 p4 #de74 ,the three axes a1 b1 c 7,fig4 #aa7 are parallel respectively to the two bisectrices of the directions of single refraction1 and the direction perpendicular to the plane containing those directions4 #e4 ,monoclinic4--,such crystals can be referred to three axes1 one of which is at right angles to the other two1 which are1 however1 themselves not at right angles4 ,spodumene 7kunzite7 and some moonstone crystallize in this system4 ,crystals belonging to this system are doubly refractive and biaxial1 but in this case the first axis alone is parallel to one of the principal optical directions4 #f4 ,triclinic4--,such crystals have no edges at right angles1 and the optical characters are not immediately related to the crystalline form4 ,some moonstone crystallizes in this system4 ,illustration3 ,,fig4 #ab4--,twinned ,octahedron4 ,crystals are often not single separate individuals4 ,for instance1 diamond and spinel are found in flat triangular crystals with their girdles cleft at the corners 7,fig4 #ab74 ,each of such crystals is really composed of portions of two similar octahedra1 which are placed together in such a way that each is a reflection of the other4 ,such composite crystals are called twins or macles4 ,sometimes the twinning is repeated1 and the individuals may be so minute as to call for a microscope for their perception4 ,a composite structure may also result from the conjunction of numberless minute individuals without any definite orientation1 as in the case of chalcedony and agate4 ,so by supposing the individuals to become infinitesimally small1 we pass to a glass-like substance4 ,it is often a peculiarity of crystals of a species to display a typical combination of natural faces4 ,such a combination is known as the habit of the species1 and is often of service for the purpose of identifying stones before they are cut4 ,thus1 a habit of diamond and spinel is an octahedron1 often twinned1 of garnet a dodecahedron1 of emerald a flat-ended hexagonal prism1 and so on4 ,it is one of the most interesting and remarkable features connected with crystallization that the composition and the physical characters--for instance1 the refractive indices and specific gravity--may1 without any serious disturbance of the molecular arrangement1 vary considerably owing to the more or less complete replacement of one element by another closely allied to it4 ,that is the cause of the range of the physical characters which has been observed in such species as tourmaline1 peridot1 spinel1 etc4 ,the principal replacements in the case of the gem-stones are ferric oxide1 ,fe#b,o#c1 by alumina1 ,al#b,o#c1 and ferrous oxide1 ,fe,o1 by magnesia1 ,mg,o4 ,a list of the principal gem-stones1 arranged by their chemical composition1 is given in ,table ,i at the end of the book4 ,,chapter ,,iii ,,reflection1 ,,refraction1 ,,and ,,dispersion ,it is obvious that1 since a stone suitable for ornamental use must appeal to the eye1 its most important characters are those which depend upon light2 indeed1 the whole art of the lapidary consists in shaping it in such a way as to show these qualities to the best advantage4 ,to understand why certain forms are given to a cut stone1 it is essential for us to ascertain what becomes of the light which falls upon the surface of the stone2 further1 we shall find that the action of a stone upon light is of very great help in distinguishing the different species of gem-stones4 ,the phenomena displayed by light which impinges upon the surface separating two media#a are very similar in character1 whatever be the nature of the media4 ,ordinary experience with a plane mirror tells us that1 when light is returned1 or reflected1 as it is usually termed1 from a plane or flat surface1 there is no alteration in the size of objects viewed in this way1 but that the right and the left hands are interchanged3 our right hand becomes the left hand in our reflection in the mirror4 ,we notice1 further1 that our reflection is apparently just as far distant from the mirror on the farther side as we are on this side4 ,in ,fig4 #ac ,,mm is a section of the mirror1 and ,o is the image of the hand ,o as seen in the mirror4 ,light from ,o reaches the eye ,e by way of m1 but it appears to come from ,o4 ,since ,,oo is perpendicular to the mirror1 and ,o and ,o lie at equal distances from it1 it follows from elementary geometry that the angle i1 which the reflected ray makes with mn1 the normal to the mirror1 is equal to i1 the angle which the incident ray makes with the same direction4 ,illustration3 ,,fig4 #ac4--,reflection at a ,plane ,mirror4 ,again1 everyday experience tells us that the case is less simple when light actually crosses the bounding surface and passes into the other medium4 ,thus1 if we look down into a bath filled with water1 the bottom of the bath appears to have been raised up1 and a stick plunged into the water seems to be bent just at the surface1 except in the particular case when it is perfectly upright4 ,since the stick itself has not been bent1 light evidently suffers some change in direction as it passes into the water or emerges therefrom4 ,the passage of light from one medium to another was studied by ,snell in the seventeenth century1 and he enunciated the following laws3-- #a4 ,the refracted ray lies in the plane containing the incident ray and the normal to the plane surface separating the two media4 ,it will be noticed that the reflected ray obeys this law also4 #b4 ,the angle r1 which the refracted ray makes with the normal1 is related to the angle i1 which the incident ray makes with the same direction1 by the equation n sin i n sin r1 7a7 where n and n are constants for the two media which are known as the indices of refraction1 or the refractive indices4 ,this simple trigonometrical relation may be expressed in geometrical language4 ,suppose we cut a plane section through the two media at right angles to the bounding plane1 which then appears as a straight line1 ,,sos 7,fig4 #ad71 and suppose that ,,io represents the direction of the incident ray2 then ,snell's first law tells us that the refracted ray ,,or will also lie in this plane4 ,draw the normal ,,non1 and with centre ,o and any radius describe a circle intersecting the incident and refracted rays in the points a and b respectively2 let drop perpendiculars ac and bd on to the normal ,,non4 ,then we have n4ac n4bd1 whence we see that if n be greater than n1 ac is less than bd1 and therefore when light passes from one medium into another which is less optically dense1 in its passage across the boundary it is bent1 or refracted1 away from the normal4 ,illustration3 ,,fig4 #ad4--,refraction across a ,plane ,surface4 ,we see1 then1 that when light falls on the boundary of two different media1 some is reflected in the first and some is refracted into the second medium4 ,the relative amounts of light reflected and refracted depend on the angle of incidence and the refractive indices of the media4 ,we shall return to this point when we come to consider the lustre of stones4 ,we will proceed to consider the course of rays at different angles of incidence when light passes out from a medium into one less dense--for instance1 from water into air4 ,corresponding to light with a small angle of incidence such as ,i#a,o 7,fig4 #ae71 some of it is reflected in the direction ,,oi#a and the remainder is refracted out in the direction ,,or#a4 ,similarly1 for the ray ,i#b,o some is reflected along ,,oi#b and some refracted along ,,or#b4 ,since1 in the case we have taken1 the angle of refraction is greater than the angle of incidence1 the refracted ray corresponding to some incident1 ray ,ic,o will graze the bounding surface1 and corresponding to a ray beyond it1 such as ,i#c,o1 which has a still greater angle of incidence1 there is no refracted ray1 and all the light is wholly or totally reflected within the dense medium4 ,the critical angle ,ic,,on1 which is called the angle of total-reflection1 is very simply related to the refractive indices of the two media2 for1 since r is now a right angle1 sin r #a1 and equation 7a7 becomes n sin i n 7b7 ,hence1 if the angle of total-reflection is measured and one of the indices is known1 the other can easily be calculated4 ,illustration3 ,,fig4 #ae4--,total-,reflection4 ,the phenomenon of total-reflection may be appreciated if we hold a glass of water above our head1 and view the light of a lamp on a table reflected from the under surface of the water4 ,this reflection is incomparably more brilliant than that given by the upper surface4 ,the refractive index of air is taken as unity2 strictly1 it is that of a vacuum1 but the difference is too small to be appreciated even in very delicate work4 ,every substance has different indices for light of different colour1 and it is customary to take as the standard the yellow light of a sodium flame4 ,this happens to be the colour to which our eyes are most sensitive1 and a flame of this kind is easily prepared by volatilizing a little bicarbonate of soda in the flame of a bunsen burner4 ,a survey of ,table ,,iii at the end of the book shows clearly how valuable a measurement of the refractive index is for determining the species to which a cut stone belongs4 ,the values found for different specimens of the species do in cases vary considerably owing to the great latitude possible in the chemical constitution due to the isomorphous replacement of one element by another4 ,some variation in the index may even occur in different directions within the same stone2 it results from the remarkable property of splitting up a beam of light into two beams1 which is possessed by many crystallized substances4 ,this forms the subject of a later chapter4 ,upon the fact that the refractive index of a substance varies for light of different colours depends such familiar phenomena as the splendour of the rainbow and the #8fire' of the diamond4 ,when white light is refracted into a stone it no longer remains white1 but is split up into a spectrum4 ,except in certain anomalous substances the refractive index increases progressively as the wave-length of the light decreases1 and consequently a normal spectrum is violet at one end and passes through green and yellow to red at the other end4 ,the width of the spectrum1 which may be measured by the difference between the refractive indices for the extreme red and violet rays1 also varies1 though on the whole it increases with the refractive index4 ,it is the dispersion1 as this difference is termed1 that determines the #8fire'--a character of the utmost importance in colourless transparent stones1 which1 but for it1 would be lacking in interest4 ,diamond excels all colourless stones in this respect1 although it is closely followed by zircon1 the colour of which has been driven off by heating2 it is1 however1 surpassed by two coloured species3 sphene1 which is seldom seen in jewellery1 and demantoid1 the green garnet from the ,urals1 which often passes under the misnomer #8olivine4' ,the dispersion of the more prominent species for the ,b and ,g lines of the solar spectrum is given in ,table ,,iv at the end of the book4 ,we will now proceed to discuss methods that may be used for the measurement of the refractive indices of cut stones4 ,,chapter ,,iv ,,measurement ,,of ,,refractive ,,indices ,the methods available for the measurement of refractive indices are of two kinds1 and make use of two different principles4 ,the first1 which is based upon the very simple relation found in the last chapter to subsist at total-reflection1 can be used with ease and celerity1 and is best suited for discriminative purposes2 but it is restricted in its application4 ,the second1 which depends on the measurement of the angle between two facets and the minimum deviation experienced by a ray of light when traversing a prism formed by them1 is more involved1 entails the use of more elaborate apparatus1 and takes considerable time1 but it is less restricted in its application4 7#a7 ,,the ,,method ,,of ,,total-,,reflection ,we see from equation b 7p4 #ah71 connecting the angle of total-reflection with the refractive indices of the adjacent media1 that1 if the denser medium be constant1 the indices of all less dense media may be easily determined from a measurement of the corresponding critical angle4 ,in all refractometers the constant medium is a glass with a high refractive index4 ,some instruments have rotatory parts1 by means of which this angle is actually measured4 ,such instruments give very good results1 but suffer from the disadvantages of being neither portable nor convenient to handle1 and of not giving a result without some computation4 ,illustration3 ,,fig4 #af4--,refractometer 7actual size74 ,for use in the identification of cut stones1 a refractometer with a fixed scale1 such as that 7,fig4 #af7 devised by the author1 is far more convenient4 ,in order to facilitate the observations1 a totally reflecting prism has been inserted between the two lenses of the eyepiece4 ,the eyepiece may be adjusted to suit the individual eyesight2 but for observers with exceptionally long sight an adapter is provided1 which permits the eyepiece being drawn out to the requisite extent4 ,the refractometer must be held in the manner illustrated in ,fig4 #ag1 so that the light from a window or other source of illumination enters the instrument by the lenticular opening underneath4 ,good1 even illumination of the field may also very simply be secured by reflecting light into the instrument from a sheet of white paper laid on a table4 ,on looking down the eyepiece we see a scale 7,fig4 #ah71 the eyepiece being1 if necessary1 focused until the divisions of the scale are clearly and distinctly seen4 ,suppose1 for experiment1 we smear a little vaseline or similar fatty substance on the plane surface of the dense glass1 which just projects beyond the level of the brass plate embracing it4 ,the field of view is now no longer uniformly illuminated1 but is divided into two parts 7,fig4 #ai73 a dark portion above1 which terminates in a curved edge1 apparently green in colour1 and a bright portion underneath1 which is composed of totally reflected light4 ,if we tilt the instrument downwards so that light enters the instrument from above through the vaseline we find that the portions of the field are reversed1 the dark portion being underneath and terminated by a red edge4 ,it is possible so to arrange the illumination that the two portions are evenly lighted1 and the common edge becomes almost invisible4 ,it is therefore essential for obtaining satisfactory results that the plate and the dense glass be shielded from the light by the disengaged hand4 ,the shadow-edge is curved1 and is1 indeed1 an arc of a circle1 because spherical surfaces are used in the optical arrangements of the refractometer2 by the substitution of cylindrical surfaces it becomes straight1 but sufficient advantage is not secured thereby to compensate for the greatly increased complexity of the construction4 ,the shadow-edge is coloured1 because the relative dispersion1 nv nr ------------------------------ 7nv and nr being the refractive indices for n the extreme violet and red rays respectively71 of the vaseline differs from that of the dense glass4 ,the dispersion of the glass is very high1 and exceeds that of any stone for which it can be used4 ,certain oils have1 however1 nearly the same relative dispersion1 and the edges corresponding to them are consequently almost colourless4 ,a careful eye will perceive that the coloured shadow-edge is in reality a spectrum1 of which the violet end lies in the dark portion of the field and the red edge merges into the bright portion4 ,the yellow colour of a sodium flame1 which1 as has already been stated1 is selected as the standard for the measurement of refractive indices1 lies between the green and the red1 and the part of the spectrum to be noted is at the bottom of the green1 and practically1 therefore1 at the bottom of the shadow1 because the yellow and red are almost lost in the brightness of the lower portion of the field4 ,if a sodium flame be used as the source of illumination1 the shadow-edge becomes a sharply defined line4 ,the scale is so graduated and arranged that the reading where this line crosses the scale gives the corresponding refractive index1 the reading1 since the line is curved1 being taken in the middle of the field on the right-hand side of the scale4 ,the refractometer therefore gives at once1 without any intermediate calculation1 a value of the refractive index to the second place of decimals1 and a skilled observer may1 by estimating the tenths of the intervals between successive divisions1 arrive at the third place2 to facilitate this estimation the semi-divisions beyond #afej have been inserted4 ,the range extends nearly to #ahjj2 for any substance with a higher refractive index the field is dark as far as the limit at the bottom4 ,illustration3 ,,fig4 #ag4--,method of ,using the ,refractometer4 ,illustration3 ,,fig4 #ah4--,scale of the ,refractometer4 ,illustration3 ,,fig4 #ai4--,shadow-edge given by a singly refractive ,substance4 ,a fat1 or a liquid1 wets the glass1 i4e4 comes into intimate contact with it1 but if a solid substance be tested in the same way1 a film of air would intervene and entirely prevent an observation4 ,to displace it1 a drop of some liquid which is more highly refractive than the substance under test must first be applied to the plane surface of the dense glass4 ,the most convenient liquid for the purpose is methylene iodide1 ,,ch#b;,i#b1 which1 when pure1 has at ordinary room temperatures a refractive index of #agdb4 ,it is almost colourless when fresh1 but turns reddish brown on exposure to light4 ,if desired1 it may be cleared in the manner described below 7p4 #ff71 but the film of liquid actually used is so thin that this precaution is scarcely necessary4 ,if we test a piece of ordinary glass--one of the slips used by microscopists for covering thin sections is very convenient for the purpose--first applying a drop of methylene iodide to the plane surface of the dense glass of the refractometer 7,fig4 #bj71 we notice a coloured shadow-edge corresponding to the glass-slip at about #aecj and another1 almost colourless1 at #agdb1 which corresponds to the liquid4 ,if the solid substance which is tested is more highly refractive than methylene iodide1 only the latter of the shadow-edges is visible1 and we must utilize some more refractive liquid4 ,we can1 however1 raise the refractive index of methylene iodide by dissolving sulphur#b in it2 the refractive index of the saturated liquid lies well beyond #ahjj and the shadow-edge corresponding to it1 therefore1 does not come within the range of the refractometer4 ,the pure and the saturated liquids can be procured with the instrument1 the bottles containing them being japanned on the outside to exclude light and fitted with dipping-stoppers1 by means of which a drop of the liquid required is easily transferred to the surface of the glass of the instrument4 ,so long as the liquid is more highly refractive than the stone1 or whatever may be the substance under examination1 its precise refractive index is of no consequence4 ,the facet used in the test must be flat1 and must be pressed firmly on the instrument1 so that it is truly parallel to the plane surface of the dense glass2 for good results1 moreover1 it must be bright4 ,illustration3 ,,fig4 #bj4--;,faceted ,stone in ,position on the ,refractometer4 ,illustration3 ,,fig4 #ba4--,shadow-edges given by a doubly refractive substance4 ,we have so far assumed that the substance which we are testing is simple and gives a single shadow-edge2 but1 as may be seen from ,table ,v1 many of the gem-stones are doubly refractive1 and such will1 in general1 show in the field of the refractometer two distinct shadow-edges more or less widely separated4 ,suppose1 for example1 we study the effect produced by a peridot1 which displays the phenomenon to a marked degree4 ,if we revolve the stone so that the facet under observation remains parallel to the plane surface of the dense glass of the refractometer and in contact with it1 we notice that both the shadow-edges in general move up or down the scale4 ,in particular cases1 depending upon the relation of the position of the facet selected to the crystalline symmetry1 one or both of them may remain fixed1 or one may even move across the other4 ,but whatever facet of the stone be used for the test1 and however variable be the movements of the shadow-edges1 the highest and lowest readings obtainable remain the same2 they are the principal indices of refraction1 such as are stated in ,table ,,iii at the end of the book1 and their difference measures the maximum amount of double refraction possessed by the stone4 ,the procedure is therefore simplicity itself2 we have merely to revolve the stone on the instrument1 usually through not more than a right angle1 and note the greatest and least readings4 ,it will be noticed that the shadow-edges cross the scale symmetrically in the critical and skewwise in intermediate positions4 ,fig4 #ba represents the effect when the facet is such as to give simultaneously the two readings required4 ,the shadow-edges a and b1 which are coloured in white light1 correspond to the least and greatest respectively of the principal refractive indices1 while the third shadow-edge1 which is very faint1 corresponds to the liquid used--methylene iodide4 ,it is possible1 as we shall see in a later chapter1 to learn from the motion1 if any1 of the shadow-edges something as to the character of the double refraction4 ,since1 however1 each shadow-edge is spectral in white light1 they will not be distinctly separate unless the double refraction exceeds the relative dispersion4 ,topaz1 for instance1 appears in white light to yield only a single shadow-edge1 and may thus easily be distinguished from tourmaline1 in which the double refraction is large enough for the separation of the two shadow-edges to be clearly discerned4 ,in sodium light1 however1 no difficulty is experienced in distinguishing both the shadow-edges given by substances with small amount of double refraction1 such as chrysoberyl1 quartz1 and topaz1 and a skilled observer may detect the separation in the extreme instances of apatite1 idocrase1 and beryl4 ,the shadow-edge corresponding to the greater refractive index is always less distinct1 because it lies in the bright portion of the field4 ,if the stone or its facet be small1 it must be moved on the plane surface of the dense glass until the greatest possible distinctness is imparted to the edge or edges4 ,if it be moved towards the observer from the further end1 a misty shadow appears to move down the scale until the correct position is reached1 when the edges spring into view4 ,any facet of a stone may be utilized so long as it is flat1 but the table-facet is the most convenient1 because it is usually the largest1 and it is available even when the stone is mounted4 ,that the stone need not be removed from its setting is one of the great advantages of this method4 ,the smaller the stone the more difficult it is to manipulate2 caution especially must be exercised that it be not tilted1 not only because the shadow-edge would be shifted from its true position and an erroneous value of the refractive index obtained1 but also because a corner or edge of the stone would inevitably scratch the glass of the instrument1 which is far softer than the hard gem-stones4 ,methylene iodide will in time attack and stain the glass1 and must therefore be wiped off the instrument immediately after use4 7#b7 ,,the ,,method ,,of ,,minimum ,,deviation ,if the stone be too highly refractive for a measurement of its refractive index to be possible with the refractometer just described1 and it is desired to determine this constant1 recourse must be had to the prismatic method1 for which purpose an instrument known as a goniometer#c is required4 ,two angles must be measured2 one the interior angle included between a suitable pair of facets1 and the other the minimum amount of the deviation produced by the pair upon a beam of light traversing them4 ,illustration3 ,,fig4 #bb4--,path at ,minimum ,deviation of a ,ray traversing a ,prism formed of two ,facets of a ,cut ,stone4 ,fig4 #bb represents a section of a step-cut stone perpendicular to a series of facets with parallel edges2 t is the table1 and a1 b1 c1 are facets on the culet side4 ,the path of light traversing the prism formed by the pair of facets1 t and b1 is indicated4 ,suppose that ,a is the interior angle of the prism1 i the angle of incidence of light at the first facet and i the angle of emergence at the second facet1 and r and r the angles inside the stone at the two facets respectively4 ,then at the first facet light has been bent through an angle i - r1 and again at the second facet through an angle i - r2 the angle of deviation1 ,d1 is therefore given by ,d i i - 7r r74 ,we have further that r r ,a1 whence it follows that ,a ,d i i4 ,if the stone be mounted on the goniometer and adjusted so that the edge of the prism is parallel to the axis of rotation of the instrument and if light from the collimator fall upon the table-facet and the telescope be turned to the proper position to receive the emergent beam1 a spectral image of the object-slit1 or in the case of a doubly refractive stone in general1 two spectral images1 will be seen in white light2 in the light of a sodium flame the images will be sharp and distinct4 ,suppose that we rotate the stone in the direction of diminishing deviation and simultaneously the telescope so as to retain an image in the field of view1 we find that the image moves up to and then away from a certain position1 at which1 therefore1 the deviation is a minimum4 ,the image moves in the same direction from this position whichever way the stone be rotated4 ,the question then arises what are the angles of incidence and refraction under these special conditions4 ,it is clear that a path of light is reversible2 that is to say1 if a beam of light traverses the prism from the facet t to the facet b it can take precisely the same path from the facet b to the facet t4 ,hence we should be led to expect that1 since experiment teaches us that there is only one position of minimum deviation corresponding to the same pair of facets1 the angles at the two facets must be equal1 i4e4 i i1 and r r4 ,it is1 indeed1 not difficult to prove by either geometrical or analytical methods that such is the case4 ,a ,a ,d ,therefore at minimum deviation r ------ and i ------------------1 and1 #b #b since sin i n sin r1 where n is the refractive index of the stone1 we have the simple relation-- ,a ,d sin ------------------ #b n -------------------------- ,a sin ------ #b ,this relation is strictly true only when the direction of minimum deviation is one of crystalline symmetry in the stone1 and holds therefore in general for all singly refractive stones1 and for the ordinary ray of a uniaxial stone2 but the values thus obtained even in the case of biaxial stones are approximate enough for discriminative purposes4 ,if then the stone be singly refractive1 the result is the index required2 if it be uniaxial1 one value is the ordinary index and the other image gives a value lying between the ordinary and the extraordinary indices2 if it be biaxial1 the values given by the two images may lie anywhere between the greatest and the least refractive indices4 ,the angle ,a must not be too large2 otherwise the light will not emerge at the second facet1 but will be totally reflected inside the stone3 on the other hand1 it must not be too small1 because any error in its determination would then seriously affect the accuracy of the value derived for the refractive index4 ,although the monochromatic light of a sodium flame is essential for precise work1 a sufficiently approximate value for discriminative purposes is obtained by noting the position of the yellow portion of the spectral image given in white light4 ,in the case of a stone such as that depicted in ,fig4 #bb images are given by other pairs of facets1 for instance ta and tc1 unless the angle included by the former is too large4 ,there might therefore be some doubt1 to which pair some particular image corresponded2 but no confusion can arise if the following procedure be adopted4 ,illustration3 ,,fig4 #bc4--;,course of ,observations in the ,method of ,minimum ,deviation4 ,the table1 or some easily recognizable facet1 is selected as the facet at which light enters the stone4 ,the telescope is first placed in the position in which it is directly opposite the collimator 7,t#j in ,fig4 #bc71 and clamped4 ,the scale is turned until it reads exactly zero1 #j or #cfj1 and clamped4 ,the telescope is released and revolved in the direction of increasing readings of the scale to the position of minimum deviation1 ,t4 ,the reading of the scale gives at once the angle of minimum deviation1 ,d4 ,the holder carrying the stone is now clamped to the scale1 and the telescope is turned to the position1 ,t#a1;in which the image given by reflection from the table facet is in the centre of the field of view2 the reading of the scale is taken4 ,the telescope is clamped1 and the scale is released and rotated until it reads the angle already found for ,d4 ,if no mistake has been made1 the reflected image from the second facet is now in the field of view4 ,it will probably not be quite central1 as theoretically it should be1 because the stone may not have been originally quite in the position of minimum deviation1 a comparatively large rotation of the stone producing no apparent change in the position of the refracted image at minimum deviation1 and further1 because1 as has already been stated1 the method is not strictly true for biaxial stones4 ,the difference in readings1 however1 should not exceed #b4 ,the reading1 ,s1 of the scale is now taken1 and it together with #ahj subtracted from the reading for the first facet1 and the value of ,a1 the interior angle between the two facets1 obtained4 ,let us take an example4 ,reading ,t 7 ,d7 #dj #da ,reading ,t#a #bfa #ce less #ahj #ahj #j -------------- #ha #ce ,reading ,s #da #cj -------------- ,d #bj #bj ,a #dj #e ,a #bj #b ,a #bj #b -------------- 7,a ,d7 #dj #bc ,log sin #dj #bc #i4haaea ,log sin #bj #b #i4ecdib -------------- ,log n #j4bgfei n #a4hijf4 ,the readings ,s and ,t are very nearly the same1 and therefore we may be sure that no mistake has been made in the selection of the facets4 ,in place of logarithm-tables we may make use of the diagram on ,plate ,,ii4 ,the radial lines correspond to the angles of minimum deviation and the skew lines to the prism angles1 and the distance along the radial lines gives the refractive index4 ,we run our eye along the line for the observed angle of minimum deviation and note where it meets the curve for the observed prism angle2 the refractive index corresponding to the point of intersection is at once read off4 ,this method has several obvious disadvantages3 it requires the use of an expensive and elaborate instrument1 an observation takes considerable time1 and the values of the principal refractive indices cannot in general be immediately determined4 ,table ,,iii at the end of the book gives the refractive indices of the gem-stones4 ,illustration3 ,,plate ,,ii ,,refractive ,,index ,,diagram ,,chapter ,v ,,lustre ,,and ,,sheen ,it has been already stated that whenever light in one medium falls upon the surface separating it from another medium some of the light is reflected within the first1 while the remainder passes out into the second medium1 except when the first is of lower refractivity than the second and light falls at an angle greater than that of total-reflection4 ,similarly1 when light impinges upon a cut stone some of it is reflected and the remainder passes into the stone4 ,what is the relative amount of reflected light depends upon the nature of the stone--its refractivity and hardness--and determines its lustre2 the greater the amount the more lustrous will the stone appear4 ,there are different kinds of lustre1 and the intensity of each depends on the polish of the surface4 ,from a dull1 i4e4 an uneven1 surface the reflected light is scattered1 and there are no brilliant reflections4 ,all gem-stones take a good polish1 and have therefore1 so long as the surface retains its polish1 considerable brilliancy2 turquoise1 on account of its softness1 is always comparatively dull4 ,the different kinds of lustre are-- 7#a7 ,adamantine1 characteristic of diamond4 7#b7 ,vitreous1 as seen on the surface of fractured glass4 7#c7 ,resinous1 as shown by resins4 ,zircon and demantoid1 the green garnet called by jewellers 8olivine10 alone among gem-stones have a lustre approaching that of diamond4 ,the remainder all have a vitreous lustre1 though varying in degree1 the harder and the more refractive species being on the whole the more lustrous4 ,some stones--for instance1 a cinnamon garnet--appear to have a certain greasiness in the lustre1 which is caused by stray reflections from inclusions or other breaks in the homogeneity of the interior4 ,a pearly lustre1 which arises from cleavage cracks and is typically displayed by the cleavage face of topaz1 would be seen in a cut stone only when flawed4 ,certain corundums when viewed in the direction of the crystallographical axis display six narrow lines of light radiating at angles of #fj from a centre in a manner suggestive of the conventional representations of stars4 ,such stones are consequently known as asterias1 or more usually star-stones--star-rubies or star-sapphires1 as the case may be1 and the phenomenon is called asterism4 ,these stones have not a homogeneous structure1 but contain tube-like cavities regularly arranged at angles of #fj in planes at right angles to the crystallographical axis4 ,the effect is best produced when the stones are cut en cabochon perpendicular to that axis4 ,chatoyancy is a somewhat similar phenomenon1 but in this case the fibres or cavities are parallel to a single direction1 and a single broadish band is displayed at right angles to it4 ,cat's-eyes1 as these stones are termed1 are cut en cabochon parallel to the fibres4 ,the true cat's-eye 7,plate ,,xxix1 ,fig4 #a7 is a variety of chrysoberyl1 but the term is also often applied to quartz showing a similar appearance4 ,the latter is really a fibrous mineral1 such as asbestos1 which has become converted into silica4 ,the beautiful tiger's-eye from ,south ,africa is a silicified crocidolite1 the original blue colour of which has been altered by oxidation to golden brown4 ,recently tourmalines have been discovered which are sufficiently fibrous in structure to display an effective chatoyancy4 ,the milky sheen of moonstone 7,plate ,,xxix1 ,fig4 #d7 owes its effect to reflections from twin lamell4 ,the wonderful iridescence which is the glory of opal1 and is therefore termed opalescence1 arises from a structure which is peculiar to that species4 ,opal is a solidified jelly2 on cooling it has become riddled with extremely thin cracks1 which were subsequently filled with similar material of slightly different refractivity1 and thus it consists of a series of films4 ,at the surface of each film interference of light takes place just as at the surface of a soap-bubble1 and the more evenly the films are spaced apart the more uniform is the colour displayed1 the actual tint depending upon the thickness of the films traversed by the light giving rise to the phenomenon4 ,,chapter ,,vi ,,double ,,refraction ,the optical phenomenon presented by many gem-stones is complicated by their property of splitting up a beam of light into two with1 in general1 differing characters4 ,in this chapter we shall discuss the nature of double refraction1 as it is termed1 and methods for its detection4 ,the phenomenon is not one that comes within the purview of everyday experience4 ,so long ago as #affi a ,danish physician1 by name ,bartholinus1 noticed that a plate of the transparent mineral which at that time had recently been brought over from ,iceland1 and was therefore called 8,iceland-spar10 possessed the remarkable property of giving a double image of objects close to it when viewed through it4 ,subsequent investigation has shown that much crystallized matter is doubly refractive1 but in calcite--to use the scientific name for the species which includes ,iceland-spar--alone among common minerals is the phenomenon so conspicuous as to be obvious to the unaided eye4 ,the apparent separation of the pair of images given by a plate cut or cleaved in any direction depends upon its thickness4 ,the large mass1 upwards of two feet 7#fj cm47 in thickness1 which is exhibited at the far end of the ,mineral ,gallery of the ,british ,museum 7,natural ,history71 displays the separation to a degree that is probably unique4 ,illustration3 ,,fig4 #bd4--;,apparent doubling of the ,edges of a ,peridot when viewed through the ,table-,facet4 ,although none of the gem-stones can emulate calcite in this character1 yet the double refraction of certain of them is large enough to be detected without much difficulty4 ,in the case of faceted stones the opposite edges should be viewed through the table-facet1 and any signs of doubling noted4 ,the double refraction of sphene is so large1 viz4 #jjh1 that the doubling of the edges is evident to the unaided eye4 ,in peridot 7,fig4 #bd71 zircon 7b71 and epidote the apparent separation of the edges is easily discerned with the assistance of an ordinary lens4 ,a keen eye can detect the phenomenon even in the case of such substances as quartz with small double refraction4 ,it must1 however1 be remembered that in all such stones the refraction is single in certain directions1 and the amount of double refraction varies therefore with the direction from nil to the maximum possessed by the stone4 ,experiment with a plate of ,iceland-spar shows that the rays transmitted by it have properties differing from those of ordinary light4 ,on superposing a second plate we notice that there are now two pairs of images1 which are in general no longer of equal brightness1 as was the case before4 ,if the second plate be rotated with respect to the first1 two images1 one of each pair1 disappear1 and then the other two1 the plate having turned through a right angle between the two positions of extinction2 midway between these positions the images are all equally bright4 ,this variation of intensity implies that each of the rays emerging from the first plate has acquired a one-sided character1 or1 as it is usually expressed1 has become plane-polarized1 or1 shortly1 polarized4 ,illustration3 ,,fig4 #be4--,wave-,motion4 ,before the discovery of the phenomenon of double refraction the foundation of the modern theory of light had been laid by the genius of ,huygens4 ,according to this theory light is the result of a wave-motion 7,fig4 #be7 in the ether1 a medium that pervades the whole of space whether occupied by matter or not1 and transmits the wave-motion at a rate varying with the matter with which it happens to coincide4 ,such a medium has been assumed because it explains satisfactorily all the phenomena of light1 but it by no means follows that it has a concrete existence4 ,indeed1 if it has1 it is so tenuous as to be imperceptible to the most delicate experiments4 ,the wave-motion is similar to that observed on the surface of still water when disturbed by a stone flung into it4 ,the waves spread out from the source of disturbance2 but1 although the waves seem to advance1 the actual particles of water merely move up and down1 and have no motion at all in the direction in which the waves are moving4 ,if we imagine similar motion to take place in any plane and not only the horizontal1 we form some idea of the nature of ordinary light4 ,but after passing through a plate of ,iceland-spar1 light no longer vibrates in all directions1 but in each beam the vibrations are parallel to a particular plane1 the two planes being at right angles4 ,the exact relation of the direction of the vibrations to the plane of polarization is uncertain1 although it undoubtedly lies in the plane containing the direction of the ray of light and the perpendicular to the plane of polarization4 ,the waves for different colours differ in their length1 i4e4 in the distance1 #b bb 7,fig4 #be71 from crest to crest1 while the velocity1 which remains the same for the same medium1 is proportional to the wave-length4 ,the intensity of the light varies as the square of the amplitude of the wave1 i4e4 the height1 ab1 of the crest from the mean level4 ,various methods have been proposed for obtaining polarized light4 ,thus ,seebeck found in #ahac that a plate of brown tourmaline cut parallel to the crystallographic axis and of sufficient thickness 7cf4 p4 #aa7 transmits only one ray1 the other being entirely absorbed within the plate4 ,another method was to employ a glass plate to reflect light at a certain critical angle4 ,the most efficient method1 and that in general use at the present day1 is due to the invention of ,nicol4 ,a rhomb of ,iceland-spar 7,fig4 #bf71 of suitable length1 is sliced along the longer diagonal1 dd1 and the halves are cemented together by means of canada balsam4 ,one ray1 ioo1 is totally reflected at the surface separating the mineral and the cement1 and does not penetrate into the other half2 while the other ray1 iee1 is transmitted with almost undiminished intensity4 ,such a rhomb is called a ,nicol's prism after its inventor1 or briefly1 a nicol4 ,illustration3 ,,fig4 #bf4--,nicol's ,prism4 ,if one nicol be placed above another and their corresponding principal planes be at right angles no light is transmitted through the pair4 ,in the polarizing microscope one such nicol1 called the polarizer1 is placed below the stage1 and the other1 called the analyser1 is either inserted in the body of the microscope or placed above the eyepiece1 and the pair are usually set in the crossed position so that the field of the microscope is dark4 ,if a piece of glass or a fragment of some singly refractive substance be placed on the stage the field still remains dark2 but in case of a doubly refractive stone the field is no longer dark except in certain positions of the stone4 ,on rotation of the plate1 or1 if possible1 of the nicols together1 the field passes from darkness to maximum brightness four times in a complete revolution1 the relative angular intervals between these positions being right angles4 ,these positions of darkness are known as the positions of extinction1 and the plate is said to extinguish in them4 ,this test is exceedingly delicate and reveals the double refraction even when the greatest difference in the refractive indices is too small to be measured directly4 ,doubly refractive substances are of two kinds3 uniaxial1 in which there is one direction of single refraction1 and biaxial1 in which there are two such directions4 ,in the case of the former the direction of one1 the ordinary ray1 is precisely the same as if the refraction were single1 but the refractive index of the other ray varies from that of the ordinary ray to a second limiting value1 the extraordinary refractive index1 which may be either greater or less4 ,if the extraordinary is greater than the ordinary refractive index the double refraction is said to be positive2 if less1 to be negative4 ,a biaxial substance is more complex4 ,it possesses three principal directions1 viz41 the bisectrices of the directions of single refraction and the perpendicular to the plane containing them4 ,the first two correspond to the greatest and least1 and the last to the mean of the principal indices of refraction4 ,if the acute bisectrix corresponds to the least refractive index1 the double refraction is said to be positive1 and if to the greatest1 negative4 ,the relation of the directions of single refraction1 s1 to the three principal directions1 a1 b1 c1 is illustrated in ,fig4 #bg for the case of topaz1 a positive mineral4 ,the refractive indices of the rays traversing one of the principal directions have the values corresponding to the other two4 ,in the direction a we should measure the greatest and the mean of the principal refractive indices1 in the direction b the greatest and the least1 and in the direction c the mean and the least4 ,the maximum amount of double refraction is therefore in the direction b4 ,illustration3 ,,fig4 #bg4--,relation of the two ,directions of single ,refraction to the principal ,optical ,directions in a ,biaxial ,crystal4 ,in the examination of a faceted stone1 of the most usual shape1 the simplest method is to lay the large facet1 called the table1 on a glass slip and view the stone through the small parallel facet1 the culet4 ,should the latter not exist1 it may frequently happen that owing to internal reflection no light emerges through the steeply inclined facets4 ,this difficulty is easily overcome by immersing the stone in some highly refracting oil4 ,a glass plate held by hand over the stone with a drop of the oil between it and the plate serves the purpose1 and is perhaps a more convenient method4 ,a stone which does not possess a pair of parallel facets should be viewed through any pair which are nearly parallel4 ,we have stated that a plate of glass has no effect on the field4 ,suppose1 however1 it were viewed when placed between the jaws of a tightened vice and thus thrown into a state of strain1 it would then show double refraction1 the amount of which would depend on the strain4 ,natural singly refractive substances frequently show phenomena of a similar kind4 ,thus diamond sometimes contains a drop of liquid carbonic acid1 and the strain is revealed by the coloured rings surrounding the cavity which are seen when the stone is viewed between crossed nicols4 ,double refraction is also common in diamond even when there is no included matter to explain it1 and is caused by the state of strain into which the mineral is thrown on release from the enormous pressure under which it was formed4 ,other minerals which display these so-called optical anomalies1 such as fluor and garnet1 are not really quite singly refractive at ordinary temperatures2 each crystal is composed of several double refractive individuals4 ,but all such phenomena cannot be confused with the characters of minerals which extinguish in the ordinary way1 since the stone will extinguish in small patches and these will not be dark all at the same time2 further1 the double refraction is small1 and on revolving the stone between crossed nicols the extinction is not sharp4 ,paste stones are sometimes in a state of strain1 and display slight1 but general1 double refraction4 ,hence the existence of double refraction does not necessarily prove that the stone is real and not an imitation4 ,stones may be composed of two or more individuals which are related to each other by twinning1 in which case each individual would in general extinguish separately4 ,such individuals would be larger and would extinguish more sharply than the patches of an anomalous stone4 ,illustration3 ,,fig4 #bh4--;,interference of ,light4 ,an examination in convergent light is sometimes of service4 ,an auxiliary lens is placed over the condenser so as to converge the light on to the stone4 ,light now traverses the stone in different directions2 the more oblique the direction the greater the distance traversed in the stone4 ,if it be doubly refractive1 in any given direction there will be in general two rays with differing refractive indices and the resulting effect is akin to the well-known phenomenon of ,newton's rings1 and is an instance of what is termed interference4 ,it may be mentioned that the interference of light 7,fig4 #bh7 explains such common phenomena as the colours of a soap-bubble1 the hues of tarnished steel1 the tints of a layer of oil floating on water1 and so on4 ,light1 after diverging from the stone1 comes to focus a little beneath the plane in which the image of the stone is formed4 ,an auxiliary lens must1 therefore1 be inserted to bring the focal planes together1 so that the interference picture may be viewed by means of the same eyepiece4 ,if a uniaxial crystal be examined along the crystallographic axis in convergent light an interference picture will be seen of the kind illustrated on ,plate ,,iii4 ,the arms of a black cross meet in the centre of the field1 which is surrounded by a series of circular rings1 coloured in white light4 ,rotation of the stone about the axis produces no change in the picture4 ,illustration3 ,,plate ,,iii #a4 ,,uniaxial #b4 ,,uniaxial 7,circular ,polarization7 #c4 ,,biaxial 7,crossed ,brushes7 #d4 ,,biaxial 7,hyperbolic ,brushes7 ,,interference ,,figures ,a biaxial substance possesses two directions 7the optic axes7 along which a single beam is transmitted4 ,if such a stone be examined along the line bisecting the acute angle between the optic axes 7the acute bisectrix7 an interference picture#d will be seen which in particular positions of the stone with respect to the crossed nicols takes the forms illustrated on ,plate ,,iii4 ,as before1 there is a series of rings which are coloured in white light2 they1 however1 are no longer circles but consist of curves known as lemniscates1 of which the figure of #h is a special form4 ,instead of an unchangeable cross there are a pair of black 8brushes0 which in one position of the stone are hyperbol1 and in that at right angles become a cross4 ,on rotating the stone we find that the rings move with it and are unaltered in form1 whereas the brushes revolve about two points1 called the 8eyes10 where the optic axes emerge4 ,if the observation were made along the obtuse bisectrix the angle between the optic axes would probably be too large for the brushes to come into the field1 and the rings might not be visible in white light1 though they would appear in monochromatic light4 ,in the case of a substance like sphene the figure is not so simple1 because the positions of the optic axes vary greatly for the different colours and the result is exceedingly complex2 in monochromatic light1 however1 the usual figure is visible4 ,it would probably not be possible in the case of a faceted stone to find a pair of faces perpendicular to the required direction4 ,nevertheless1 so long as a portion of the figures described is in the field of view1 the character of the double refraction1 whether uniaxial or biaxial1 may readily be determined4 ,there is yet another remarkable phenomenon which must not be passed over4 ,certain substances1 of which quartz is a conspicuous example and in this respect unique among the gem-stones1 possess the remarkable property of rotating the plane of polarization of a ray of light which is transmitted parallel to the optic axis4 ,if a plate of quartz be cut at right angles to the axis and placed between crossed nicols in white light1 the field will be coloured1 the hue changing on rotation of one nicol with respect to the other4 ,examination in monochromatic light shows that the field will become dark after a certain rotation of the one nicol with respect to the other1 the amount of which depends on the thickness of the plate4 ,if the plate be viewed in convergent light1 an interference picture is seen as illustrated on ,plate ,,iii1 which is similar to1 and yet differs in some important particulars from the ordinary interference picture of a uniaxial stone4 ,the cross does not penetrate beyond the innermost ring and the centre of the field is coloured in white light4 ,if a stone shows such a picture1 it may be safely assumed to be quartz4 ,it is interesting to note that minerals which possess this property have a spiral arrangement of the constituent atoms4 ,it has already been remarked 7p4 #bh7 that if a faceted doubly refractive stone be rotated with one facet always in contact with the dense glass of the refractometer the pair of shadow-edges that are visible in the field move up or down the scale in general from or to maximum and minimum positions4 ,the manner in which this movement takes place depends upon the character of the double refraction and the position of the facet under observation with regard to the optical symmetry of the stone4 ,in the case of a uniaxial stone1 if the facet be perpendicular to the crystallographic axis1 i4e4 the direction of single refraction1 neither of the shadow-edges will move4 ,if the facet be parallel to that direction1 one shadow-edge will move up and coincide with the other1 which remains invariable in position1 and away from it to a second critical position2 the latter gives the value of the extraordinary refractive index1 and the invariable shadow-edge corresponds to the ordinary refractive index4 ,this phenomenon is displayed by the table-facet of most tourmalines1 because for reasons given above 7p4 #aa7 they are as a rule cut parallel to the crystallographic axis4 ,in the case of facets in intermediate positions1 the shadow-edge corresponding to the extraordinary refractive index moves1 but not to coincidence with the invariable shadow-edge4 ,the case of a biaxial stone is more complex4 ,if the facet be perpendicular to one of the principal directions one shadow-edge remains invariable in position1 corresponding to one of the principal refractive indices1 whilst the other moves between the critical values corresponding to the remaining two of the principal refractive indices4 ,in the interesting case in which the facet is parallel to the two directions of single refraction1 the second shadow-edge moves across the one which is invariable in position4 ,in intermediate positions of the facet both shadow-edges move1 and give therefore critical values4 ,of the intermediate pair1 i4e4 the lower maximum and the higher minimum1 one corresponds to the mean principal refractive index1 and the other depends upon the relation of the facet to the optical symmetry4 ,if it is desired to distinguish between them1 observations must be made on a second facet2 but for discriminative purposes such exactitude is unnecessary1 since the least and the greatest refractive indices are all that are required4 ,the character of the refraction of gem-stones is given in ,table ,v at the end of the book4 ,,chapter ,,vii ,,absorption ,,effects3 ,,colour1 ,,dichroism1 ,,etc4 ,when white light passes through a cut stone1 colour effects result which arise from a variety of causes4 ,the most obvious is the fundamental colour of the stone1 which is due to its selective absorption of the light passing through it1 and would characterize it before it was cut4 ,intermingled with the colour in a transparent stone is the dispersive effect known as #8fire1' which has already been discussed 7p4 #bj74 ,in many instances the want of homogeneity is responsible for some peculiar effects such as opalescence1 chatoyancy1 and asterism4 ,these phenomena will now be considered in fuller detail4 ,,colour ,all substances absorb light to some extent4 ,if the action is slight and affects equally the whole of the visible spectrum1 the stone appears white or colourless4 ,usually some portion is more strongly absorbed than the rest1 and the stone seems to be coloured4 ,what is the precise tint depends not only upon the portions transmitted through the stone1 but also upon their relative intensities4 ,the eye1 unlike the ear1 has not the power of analysis and it cannot of itself determine how a composite colour has been made up4 ,indeed1 so far as it is concerned1 any colour may be exactly matched by compounding in certain proportions three simple primary colours--red1 yellow1 and violet4 ,alexandrite1 a variety of chrysoberyl1 is a curious and instructive case4 ,the balance in the spectrum of light transmitted through it is such that1 whereas in daylight such stones appear green1 in artificial light1 especially in gas-light1 they are a pronounced raspberry-red 7,plate ,,xxvii1 ,figs4 #aa1 #ac74 ,the phenomenon is intensified by the strong dichroism characteristic of this species4 ,the colour is the least reliable character that may be employed for the identification of a stone1 since it varies considerably in the same species1 and often results from the admixture of some metallic oxide1 which has no essential part in the chemical composition and is present in such minute quantities as to be almost imperceptible by analysis4 ,who would1 for instance1 imagine from their appearance that stones so markedly diverse in hue as ruby and sapphire were really varieties of the same species1 corundum8 ,again1 quartz1 in spite of the simplicity of its composition1 displays extreme differences of tint4 ,nevertheless1 certain varieties do possess a distinctive colour1 emerald being the most striking example1 and in other cases the trained eye can appreciate certain characteristic subtleties of shade4 ,at any rate1 the colour is the most obvious of the physical characters1 and serves to provide a rough division of the species1 and accordingly in ,table ,,ii at the end of the book the gem-stones are arranged by their usual tints4 ,,dichroism ,the two rays into which a doubly refractive stone splits up a ray of light are often differently absorbed by it1 and in consequence appear on emergence differently coloured2 such stones are said to be dichroic4 ,the most striking instance is a deep-brown tourmaline1 which1 except in very thin sections1 is quite opaque to the ordinary ray4 ,the light transmitted by a plate cut parallel to the crystallographic axis is therefore plane-polarized2 before the invention by ,nicol of the prism of ,iceland-spar known by his name this was the ordinary method of obtaining light of this character 7cf4 p4 #dc74 ,again1 in the case of kunzite and cordierite the difference in colour is so marked as to be obvious to the unaided eye2 but where the contrast is less pronounced we require the use of an instrument called a dichroscope1 which enables the twin colours to be seen side by side4 ,illustration3 ,,fig4 #bi4--;,dichroscope 7actual size74 ,illustration3 ,,fig4 #cj4--;,field of the ,dichroscope4 ,fig4 #bi illustrates in section the construction of a dichroscope4 ,the instrument consists essentially of a rhomb of ,iceland-spar1 ,s1 of such a length as to give two contiguous images 7,fig4 #cj7 of a square hole1 ,h1 in one end of the tube containing it4 ,in some instruments the terminal faces of the rhomb are ground at right angles to its length1 but usually1 as in that depicted1 prisms of glass1 ,g1 are cemented on to the two ends4 ,a cap ,c1 with a slightly larger hole1 which is circular in shape1 fits on the end of the tube1 and can be moved up and down it and revolved round it1 as desired4 ,the stone1 ,r1 to be tested may be directly attached to it by means of some kind of wax or cement in such a way that light which has traversed it passes into the window1 ,h1 of the instrument2 the cap at the same time permits of the rotation of the stone about the axis of the main tube of the instrument4 ,the dichroscope shown in the figure has a still more convenient arrangement3 it is provided with an additional attachment1 ,a1 by means of which the stone can be turned about an axis at right angles to the length of the tube1 and thus examined in different directions4 ,at the other end of the main tube is placed a lens1 ,l1 of low power for viewing the twin images3 the short tube containing it can be pushed in and out for focusing purposes4 ,many makers now place the rhomb close to the lens1 ,l1 and thereby require a much smaller piece of spar2 material suitable for optical purposes is fast growing scarce4 ,suppose that a plate of tourmaline cut parallel to its crystallographic axis is fastened to the cap and the latter rotated4 ,we should notice1 on looking through the instrument1 that in the course of a complete revolution there are two positions1 orientated at right angles to one another1 in which the tints of the two images are identical1 the positions of greatest contrast of tint being midway between4 ,if we examine a uniaxial stone in a direction at right angles to its optic axis we obtain the colours corresponding to the ordinary and the extraordinary rays4 ,in any direction less inclined to the axis we still have the colour for the ordinary ray1 but the other colour is intermediate in tint between it and that for the extraordinary ray4 ,the phenomenon presented by a biaxial stone is more complex4 ,there are three principal colours which are visible in differing pairs in the three principal optical directions2 in other directions the tints seen are intermediate between the principal colours4 ,since biaxial stones have three principal colours1 they are sometimes said to be trichroic or pleochroic1 but in any single direction they have two twin colours and show dichroism4 ,no difference at all will be shown in directions in which a stone is singly refractive1 and it is therefore always advisable to examine a stone in more than one direction lest the first happens to be one of single refraction4 ,for determinative purposes it is not necessary to note the exact shades of tint of the twin colours1 because they vary with the inherent colour of the stone1 and are therefore not constant for the same species2 we need only observe1 when the stone is tested with the dichroscope1 whether there is any variation of colour1 and1 if so1 its strength4 ,dichroism is a result of double refraction1 and cannot exist in a singly refractive stone4 ,the converse1 however1 is not true and it by no means follows that1 because no dichroism can be detected in a stone1 it is singly refractive4 ,a colourless stone1 for instance1 cannot possibly be dichroic1 and many coloured1 doubly refractive stones--for example1 zircon--exhibit no dichroism1 or so little that it is imperceptible4 ,the character is always the better displayed1 the deeper the inherent colour of the stone4 ,the deep-green alexandrite1 for instance1 is far more dichroic than the lighter coloured varieties of chrysoberyl4 ,if the stone is attached to the cap of the instrument1 the table should be turned towards it so as to assure that the light passing into the instrument has actually traversed the stone4 ,if little light enters through the opposite coign1 a drop of oil placed thereon will overcome the difficulty 7cf4 p4 #df74 ,it is also necessary1 for reasons mentioned above1 to examine the stone in directions as far as possible across the girdle also4 ,a convenient1 though not strictly accurate1 method is to lay the stone with the table facet on a table and examine the light which has entered the stone and been reflected at that facet4 ,the stone may easily be rotated on the table1 and observations thus made in different directions in the stone4 ,care must be exercised in the case of a faceted stone not to mistake the alteration in colour due to dispersion for a dichroic effect1 and the stone must be placed close to the instrument during an observation1 because otherwise the twin rays traversing the instrument may have taken sensibly different directions in the stone4 ,dichroism is an effective test in the case of ruby2 its twin colours--purplish and yellowish red--are in marked contrast1 and readily distinguish it from other red stones4 ,again1 one of the twin colours of sapphire is distinctly more yellowish than the other2 the blue spinel1 of which a good many have been manufactured during recent years1 is singly refractive1 and1 of course1 shows no difference of tint in the dichroscope4 ,table ,,vi at the end of the book gives the strength of the dichroism of the gem-stones4 ,,absorption ,,spectra ,a study of the chromatic character of the light transmitted by a coloured stone is of no little interest4 ,as was stated above1 the eye has not the power of analysing light1 and to resolve the transmitted rays into their component parts an instrument known as a spectroscope is needed4 ,the small #8direct-vision' type has ample dispersion for this purpose4 ,it is advantageous to employ by preference the diffraction rather than the prism form1 because in the former the intervals in the resulting spectrum corresponding to equal differences of wave-length are the same1 whereas in the latter they diminish as the wave-length increases and accordingly the red end of the spectrum is relatively cramped4 ,the absorptive properties of all doubly refractive coloured substances vary more or less with the direction in which light traverses them according to the amount of dichroism that they possess1 but the variation is not very noticeable unless the stone is highly dichroic4 ,if the light transmitted by a deep-coloured ruby be examined with a spectroscope it will be found that the whole of the green portion of the spectrum is obliterated 7,fig4 #ca71 while in the case of a sapphire only a small portion of the red end of the spectrum is absorbed4 ,alexandrite affords especial interest4 ,in the spectrum of the light transmitted by it1 the violet and the yellow are more or less strongly absorbed1 depending upon the direction in which the rays have passed through the stone 7,fig4 #ca71 and the transmitted light is mainly composed of two portions--red and green4 ,the apparent colour of the stone depends1 therefore1 upon which of the two predominates4 ,in daylight the resultant colour is green flecked with red and orange1 the three principal absorptive tints 7cf4 p4 #bce71 but in artificial light1 which is relatively stronger in the red portion of the spectrum1 the resultant colour is a raspberry-red1 and there is less apparent difference in the absorptive tints 7cf4 ,plate ,,xxvii1 ,figs4 #aa1 #ac74 ,illustration3 ,,fig4 #ca4--;,absorption ,spectra4 ,in all the spectra just considered1 and in all like them1 the portions that are absorbed are wide1 the passage from blackness to colour is gradual1 and the edges deliminating them are blurred4 ,in the spectra of certain zircons and in almandine garnet the absorbed portions1 or bands as they are called1 are narrow1 and1 moreover1 the transition from blackness to colour is sharp and abrupt2 such stones are therefore said to display absorption-bands4 ,church in #ahff was the first to notice the bands shown by zircon 7,fig4 #ca74 ,sorby thought they portended the existence of a new element1 to which he gave the name jargonium1 but subsequently discovered that they were caused by the presence of a minute trace of uranium4 ,a yellowish-green zircon shows the phenomenon best1 and it has all the bands shown in the figure4 ,the spectrum varies slightly but almost imperceptibly with the direction in the stone4 ,others show the bands in the yellow and green1 while others show only those in the red1 and some only one of them4 ,the bands are not confined to stones of any particular colour1 or amount of double refraction4 ,again1 many zircons show no bands at all1 so that their absence by no means precludes the stone from being a zircon4 ,almandine is characterized by a different spectrum 7,fig4 #ca74 ,the band in the yellow is the most conspicuous1 and is no doubt responsible for the purple hue of a typical almandine4 ,the spectrum varies in strength in different stones4 ,rhodolite 7p4 #bad71 a garnet lying between almandine and pyrope1 displays the same bands1 and indications of them may be detected in the spectra of pyropes of high refraction4 ,illustration3 ,,plate ,,iv ,,jewellery ,,designs ,,chapter ,,viii ,,specific ,,gravity ,it is one of our earliest experiences that different substances of the same size have often markedly different weights2 thus1 there is a great difference between wood and iron1 and still greater between wood and lead4 ,it is usual to say that iron is heavier than wood1 but the statement is misleading1 because it would be possible by selecting a large enough piece of wood to find one at least as heavy as a particular piece of iron4 ,we have1 in fact1 to compare equal volumes of the two substances1 and all ambiguity is removed if we speak of relative density or specific gravity--the former term being usually applied to liquids and the latter to solids--instead of weight or heaviness4 ,the density of water at #d ,c4 is taken as unity1 that being the temperature at which it is highest2 at other temperatures it is somewhat lower1 as will be seen from ,table ,,ix given at the end of the book4 ,the direct determination of the volume of an irregular solid presents almost insuperable difficulty2 but1 fortunately1 for finding the specific gravity it is quite unnecessary to know the volume1 as will be shown when we proceed to consider the methods in use4 ,the specific gravity of a stone is a character which is within narrow limits constant for each species1 and is therefore very useful for discriminative purposes4 ,it can be determined whatever be the shape of the stone1 and it is immaterial whether it be transparent or not2 but1 on the other hand1 the stone must be unmounted and free from the setting4 ,the methods for the determination of the specific gravity are of two kinds3 in the first a liquid is found of the same1 or nearly the same1 density as the stone1 and in the second weighings are made and the use of an accurate balance is required4 7#a7 ,,heavy ,,liquids ,experiment tells us that a solid substance floats in a liquid denser than itself1 sinks in one less dense1 and remains suspended at any level in one of precisely the same density4 ,if the stone be only slightly less dense than the liquid1 it will rise to the surface2 if it be just as slightly denser1 it will as surely sink to the bottom1 a physical fact which has added so much to the difficulty and danger of submarine manuvring4 ,if then we can find a liquid denser than the stone to be tested1 and place the latter in it1 the stone will float on the surface4 ,if we take a liquid which is less dense than the stone and capable of mixing with the heavier liquid1 and add it to the latter1 drop by drop1 gently stirring so as to assure that the density of the combination is uniformly the same throughout1 a stage is finally reached when the stone begins to move downwards4 ,it has now very nearly the density of the liquid1 and1 if we find by some means this density1 we know simultaneously the specific gravity of the stone4 ,various devices and methods are available for ascertaining the density of liquids--for instance1 ,westphal's balance2 but1 apart from the inconvenience attending such a determination1 the density of all liquids is somewhat seriously affected by changes in the temperature1 and it is therefore better to make direct comparison with fragments of substances of known specific gravity1 which are termed indicators4 ,if of two fragments differing slightly in specific gravity one floats on the surface of a uniform column of liquid and the other lies at the bottom of the tube containing the liquid1 we may be certain that the density of the liquid is intermediate between the two specific gravities4 ,such a precaution is necessary because1 if the liquid be a mixture of two distinct liquids1 the density would tend to increase owing to the greater volatility of the lighter of them1 and in any case the density is affected by change of temperature4 ,the specific gravity of stones is not much altered by variation in the temperature4 ,a more convenient variation of this method is to form a diffusion column1 so that the density increases progressively with the depth4 ,if the stone under test floats at a certain level in such a column intermediate between two fragments of known specific gravity1 its specific gravity may be found by elementary interpolation4 ,to form a column of this kind the lighter liquid should be poured on to the top of the heavier4 ,natural diffusion gives the most perfect column1 but1 being a lengthy process1 it may conveniently be quickened by gently shaking the tube1 and the column thus formed gives results sufficiently accurate for discriminative purposes4 ,by far the most convenient liquid for ordinary use is methylene iodide1 which has already been recommended for its high refraction4 ,it has1 when pure1 a density at ordinary room-temperatures of #ccbd1 and it is miscible in all proportions with benzol1 whose density is ,o#hh1 or toluol1 another hydrocarbon which is somewhat less volatile than benzol1 and whose density is about the same1 namely1 #jhf4 ,when fresh1 methylene iodide has only a slight tinge of yellow1 but it rapidly darkens on exposure to light owing to the liberation of iodine which is in a colloidal form and cannot be removed by filtration4 ,the liquid may1 however1 be easily cleared by shaking it up with any substance with which the iodine combines to form an iodide removable by filtration4 ,copper filings answer the purpose well1 though rather slow in action2 mercury may also be used1 but is not very satisfactory1 because a small amount may be dissolved and afterwards be precipitated on to the stone under test1 carrying it down to the bottom of the tube4 ,caustic potash 7potassium hydroxide7 is also recommended2 in this case the operation should preferably be carried out in a special apparatus which permits the clear liquid to be drawn off underneath1 because water separates out and floats on the surface4 ,in ,fig4 #cb three cut stones1 a quartz 7a71 a beryl 7b71 and a tourmaline 7c7 are shown floating in a diffusion column of methylene iodide and benzol4 ,although the beryl is only slightly denser than the quartz1 it floats at a perceptibly lower level4 ,these three species are occasionally found as yellow stones of very similar tint4 ,illustration3 ,,fig4 #cb4--,stones of different ,specific ,gravities floating in a ,diffusion ,column of heavy ,liquid4 ,various other liquids have been used or proposed for the same purpose1 of which two may be mentioned4 ,the first of them is a saturated solution of potassium iodide and mercuric iodide in water1 which is known after the discoverer as ,sonstadt's solution4 ,it is a clear mobile liquid with an amber colour1 having at #ab ,c4 a density of #cjhe2 it may be mixed with water to any extent1 and is easily concentrated by heating2 moreover1 it is durable and not subject to alteration of any kind2 on the other hand1 it is highly poisonous and cauterizes the skin1 not being checked by albumen2 it also destroys brass-ware by amalgamating the metal4 ,the second is ,klein's solution1 a clear yellow liquid which has at #ae ,c4 a density of #cbh4 ,it consists of the boro-tungstate of cadmium1 of which the formula is #i,,wo#c4;,b#b,o#c4b;,cd,o4#b;,h#b,o #af;,aq1 dissolved in water1 with which it may be diluted4 ,if the salt be heated1 it fuses at #ge ,c4 in its own water of crystallization to a yellow liquid1 very mobile1 with a density of #cee4 ,klein's solution is harmless1 but it cannot compare for convenience of manipulation with methylene iodide4 ,the most convenient procedure is to have at hand three glass tubes1 fitted with stoppers or corks1 to contain liquids of different densities-- 7a7 ,methylene iodide reduced to #bg2 using as indicators orthoclase #bee1 quartz #bff1 and beryl #bgd4 7b7 ,methylene iodide reduced to #ca2 indicators1 beryl #bgd and tourmaline #caj4 7c7 ,methylene iodide1 undiluted1 #ccb4 ,the pure liquid in the last tube should on no account be diluted2 but the density of the other two liquids may be varied slightly1 either by adding benzol in order to lower it1 or by allowing benzol1 which has far greater volatility than methylene iodide1 to evaporate1 or by adding methylene iodide1 in order to increase it4 ,the density of the liquids may be ascertained approximately from the indicators4 ,a glance at the table of specific gravities shows that as regards the gem-stones methylene iodide is restricted in its application1 since it can be used to test only moonstone1 quartz1 beryl1 tourmaline1 and spodumene2 opal and turquoise1 being amorphous and more or less porous1 should not be immersed in liquids1 lest the appearance of the stone be irretrievably injured4 ,methylene iodide readily serves to distinguish the yellow quartz from the true topaz1 with which jewellers often confuse it1 the latter stone sinking in the liquid2 again aquamarine floats1 but the blue topaz1 which is often very similar to it1 sinks in methylene iodide4 ,by saturating methylene iodide with iodine and iodoform1 we have a liquid 7d7 of density #cf2 a fragment of topaz1 #cee1 may be used to indicate whether the liquid has the requisite density4 ,unfortunately this saturated solution is so dark as to be almost opaque1 and is1 moreover1 very viscous4 ,its principal use is to distinguish diamond1 #cece1 from the brilliant colourless zircon1 with which1 apart from a test for hardness1 it may easily be confused4 ,it is easy to see whether the stone floats1 as it would do if a diamond4 ,to recover a stone which has sunk1 the only course is to pour off the liquid into another tube1 because it is far too dark for the position of the stone to be seen4 ,it is possible to employ a similar method for still denser stones by having recourse to ,retgers's salt1 silver-thallium nitrate4 ,this double salt is solid at ordinary room-temperatures1 but has the remarkable property of melting at a temperature1 #ge ,c41 which is well below the point of fusion of either of its constituents1 to a clear1 mobile yellow liquid1 which is miscible in any proportion with water1 and has1 when pure1 a density of #df4 ,the salt may be purchased1 or it may be prepared by mixing #ajj grams of thallium nitrate and #fd grams of silver nitrate1 or similar proportions1 in a little water1 and heating the whole over a water-bath1 keeping it constantly stirred with a glass rod until it is liquefied4 ,the two salts must be mixed in the correct proportions1 because otherwise the mixture might form other double salts1 which do not melt at so low a temperature4 ,a glance at the table of specific gravities shows that ,retgers's salt may be used for all the gem-stones with the single exception of zircon 7b74 ,there are1 however1 some objections to its use4 ,it is expensive1 and1 unless kept constantly melted1 it is not immediately available4 ,it darkens on exposure to strong sunlight like all silver salts1 stains the skin a peculiar shade of purple which is with difficulty removed1 and in fact only by abrasion of the skin1 and1 like all thallium compounds1 is highly poisonous4 ,it is convenient to have three tubes1 fitted as before with stoppers or corks1 to contain the following liquids1 when heated3-- 7e7 ,silver-thallium nitrate1 reduced to #ce2 using as indicators1 peridot or idocrase #cdj and topaz #cec4 7f7 ,silver-thallium nitrate1 reduced to #dj2 indicators1 topaz #cec and sapphire #djc4 7g7 ,silver-thallium nitrate1 undiluted1 #df4 ,the tubes must be heated in some form of water-bath2 an ordinary glass beaker serves the purpose satisfactorily4 ,the pure salt should never be diluted2 but the density of the contents of tubes 7e7 and 7f7 may be varied at will1 water being added in order to lower the density1 and concentration by means of evaporation or addition of the nitrate being employed in order to increase it4 ,to avoid the discoloration of the skin1 rubber finger-stalls may be used1 and the stones should not be handled until after they have been washed in warm water4 ,the staining may be minimized if the hands be well washed in hot water before being exposed to sunlight4 ,it is advisable to warm the stone to be tested in a tube containing water beforehand lest the sudden heating develop cracks4 ,a piece of platinum1 or1 failing that1 copper wire is of service for removing stones from the tubes2 a glass rod1 spoon-shaped at one end1 does equally well4 ,it must be noted that although ,retgers's salt is absolutely harmless to the ordinary gem-stones--with the exception of opal and turquoise1 which1 as has already been stated1 being to some extent porous1 should not be immersed in liquids--it attacks certain substances1 for instance1 sulphides and cannot be applied indiscriminately to minerals4 ,the procedure described above is intended only as a suggestion2 the method may be varied to any extent at will1 depending upon the particular requirements4 ,if such tests are made only occasionally1 a smaller number of tubes may be used4 ,thus one tube may be substituted for the two marked a and b1 the liquid contained in it being diluted as required1 and a series of indicators may be kept apart in small glass tubes4 ,on the other hand1 any one having constantly to test stones might increase the number of tubes with advantage1 and might find it useful to have at hand fragments of all the principal species in order to make direct comparison4 7#b7 ,,direct ,,weighing ,the balance which is necessary in both the methods described under this head should be capable of giving results accurate to milligrams1 i4e4 the thousandth part of a gram1 and consistent with that restriction the beam may be as short as possible so as to give rapid swings and thus shorten the time taken in the observations4 ,a good assay balance answers the purpose admirably4 ,of course1 it is never necessary to wait till the balance has come to rest4 ,the mean of the extreme readings of the pointer attached to the beam will give the position in which it would ultimately come to rest4 ,thus1 if the pointer just touches the eighth division on the right-hand side and the second on the other1 the mean position is the third division on the right-hand side 77#h #b7 #c74 ,instead of the ordinary form of chemical balance1 ,westphal's form or ,joly's spring-balance may be employed4 ,weighings are made more quickly1 but are not so accurate4 ,in refined physical work the practice known as double-weighing is employed to obviate any slight error there may be in the suspension of the balance4 ,a counterpoise which is heavier than anything to be weighed is placed in one pan1 and weighed4 ,the counterpoise is retained in its pan throughout the whole course of the weighings4 ,any substance whose weight is to be found is placed in the other pan1 and weights added till the balance swings truly again4 ,the difference between the two sets of weights evidently gives the weight of the substance4 ,balances1 however1 are so accurately constructed that for testing purposes such refined precautions are not really necessary4 ,it is immaterial in what notation the weighings are made1 so long as the same is used throughout1 but the metric system of weights1 which is in universal use in scientific work1 should preferably be employed4 ,jewellers1 however1 use carat weights1 and a subdivision to the base #b instead of decimals1 the fractions being 1 1 1 #a_/af1 #a_/cb1 #a_/fd4 ,if these weights be employed1 it will be necessary to convert these fractions into decimals1 and write #ejj1 #bej1 #abe1 #a_/af #jfb1 #a_/cb #jca1 #a_/fd #jaf4 7a7 ,hydrostatic ,weighing ,the principle of this method is very simple4 ,the stone1 the specific gravity of which is required1 is first weighed in air and then when immersed in water4 ,if ,w and ,w be these weights respectively1 then ,w ,w is evidently the weight of the water displaced by the stone and having therefore the same volume as it1 and the specific gravity is ,w therefore equal to ----------------------4 ,w ,wr ,if the method of double-weighing had been adopted1 the formula would be slightly altered4 ,thus1 suppose that c corresponds to the counterpoise1 w and w to the stone weighed in air and water respectively2 then we have ,w c w and ,w c w1 and c w therefore the specific gravity is equal to --------------------4 w w ,illustration3 ,,fig4 #cc4--;,hydrostatic ,balance4 ,some precautions are necessary in practice to assure an accurate result4 ,a balance intended for specific gravity work is provided with an auxiliary pan 7,fig4 #cc71 which hangs high enough up to permit of the stone being suspended underneath4 ,the weight of anything used for the suspension must1 of course1 be determined and subtracted from the weight found for the stone1 both when in air and when in water4 ,a piece of fine silk is generally used for suspending the stone in water1 but it should be avoided1 because the water tends to creep up it and the error thus introduced affects the first place of decimals in the case of a one-carat stone1 the value being too high4 ,a piece of brass wire shaped into a cage is much to be preferred4 ,if the same cage be habitually used1 its weight in air and when immersed in water to the customary extent in such determinations should be found once for all4 ,care must also be taken to remove all air-bubbles which cling to the stone or the cage2 their presence would tend to make the value too low4 ,the surface tension of water which makes it cling to the wire prevents the balance swinging freely1 and renders it difficult to obtain a weighing correct to a milligram when the wire dips into water4 ,this difficulty may be overcome by substituting a liquid such as toluol1 which has a much smaller surface tension4 ,as has been stated above1 the density of water at #d ,c4 is taken as unity1 and it is therefore necessary to multiply the values obtained by the density of the liquid1 whatever it be1 at the temperature of the observation4 ,in ,table ,,ix1 at the end of the book1 are given the densities of water and toluol at ordinary room-temperatures4 ,it will be noticed that a correct reading of the temperature is far more important in the case of toluol4 ,example of a ,hydrostatic ,determination of ,specific ,gravity-- ,weight of stone in air #adga gram ,weight of stone in water #ajfg #adga #adga ,specific gravity -------------------------- ---------- #adga #ajfg #jdjd4 ,allowing for the density of water at the temperature of the room1 which was #af ,c41 the specific gravity is #cfcg4 ,had no such allowance been made1 the result would have been four units too high in the third place of decimals4 ,for discriminative purposes1 however1 such refinement is unnecessary4 7b7 ,pycnometer1 or ,specific ,gravity ,bottle ,the specific gravity bottle is merely one with a fairly long neck on which a horizontal mark has been scratched1 and which is closed by a ground glass stopper4 ,the pycnometer is a refined variety of the specific gravity bottle4 ,it has two openings3 the larger is intended for the insertion of the stone and the water1 and is closed by a stopper through which a thermometer passes1 while the other1 which is exceedingly narrow1 is closed by a stopper fitting on the outside1 and is graduated to facilitate the determination of the height of the water in it4 ,the stone is weighed as in the previous method4 ,the bottle is then weighed1 and filled with water up to the mark and weighed again4 ,the stone is now introduced into the bottle1 and the surplus water removed with blotting-paper or otherwise until it is at the same level as before1 and the bottle with its contents is weighed4 ,let ,w be the weight of the stone1 w the weight of the bottle1 ,w the weight of the bottle and the water contained in it1 and ,w the weight of the bottle when containing the stone and the water4 ,then ,w w is the weight of the water filling the bottle up to the mark1 and ,w w ,w is the reduced weight of water after the stone has been inserted2 the difference1 ,w ,w ,w1 is the weight of the ,w water displaced4 ,the specific gravity is therefore ------------------------------------4 ,w ,w ,w ,as in the previous method1 this value must be multiplied by the density of the liquid at the temperature of the experiment4 ,if the method of double-weighing be adopted1 the formula will be slightly modified4 ,of the above methods1 that of heavy liquids1 as it is usually termed1 is by far the quickest and the most convenient for stones of ordinary size1 the specific gravity of which is less than the density of pure methylene iodide1 namely1 #ccbd1 and by its aid a value may be obtained which is accurate to the second place of decimals1 a result quite sufficient for a discriminative test4 ,the method is applicable no matter how small the stone may be1 and1 indeed1 for very small stones it is the only trustworthy method2 for large stones it is inconvenient1 not only because of the large quantity of liquid required1 but also on account of the difficulty in estimating with sufficient certainty the position of the centre of gravity of the stone4 ,a negative determination may be of value1 especially if attention be paid to the rate at which the stone falls through the liquid2 the denser the stone the faster it will sink1 but the rate depends also upon the shape of the stone4 ,retgers's salt is less convenient because of the delay involved in warming it and of the almost inevitable staining of the hands1 but its use presents no difficulty whatever4 ,hydrostatic weighing is always available1 unless the stone be very small1 but the necessary weighings occupy considerable time1 and care must be taken that no error creeps into the computation1 simple though it be4 ,even if everything is at hand1 a determination is scarcely possible under a quarter of an hour4 ,the third method1 which takes even longer1 is intended primarily for powdered substances1 and is not recommended for cut stones1 unless there happen to be a number of tiny ones which are known to be exactly of the same kind4 ,the specific gravities of the gem-stones are given in ,table ,,vii at the end of the book4 ,,chapter ,,ix ,,hardness ,,and ,,cleavability ,every possessor of a diamond ring is aware that diamond easily scratches window-glass4 ,if other stones were tried1 it would be found that they also scratched glass1 but not so readily1 and1 if the experiment were extended1 it would be found that topaz scratches quartz1 but is scratched by corundum1 which in its turn yields to the all-powerful diamond4 ,there is therefore considerable variation in the capacity of precious stones to resist abrasion1 or1 as it is usually termed1 in their hardness4 ,to simplify the mode of expressing this character the mineralogist ,mohs about a century ago devised the following arbitrary scale1 which is still in general use4 ,,mohs',s ,,scale ,,of ,,hardness #a4 ,talc #b4 ,gypsum #c4 ,calcite #d4 ,fluor #e4 ,apatite #f4 ,orthoclase #g4 ,quartz #h4 ,topaz #i4 ,corundum #aj4 ,diamond ,a finger-nail scratches gypsum and softer substances4 ,ordinary window-glass is slightly softer than orthoclase1 and a steel knife is slightly harder2 a hardened file approaches quartz in hardness1 and easily scratches glass4 ,by saying that a stone has hardness #g we merely mean that it will not scratch quartz1 and quartz will not scratch it4 ,the numbers indicate an order1 and have no quantitative significance whatever4 ,this is an important point about which mistakes are often made4 ,we must not1 for instance1 suppose that diamond has twice the hardness of apatite4 ,as a matter of fact1 the interval between diamond and corundum is immensely greater than that between the latter and talc1 the softest of mineral substances4 ,intermediate degrees of hardness are expressed by fractions4 ,the number #h for chrysoberyl means that it scratches topaz as easily as it itself is scratched by corundum4 ,pyrope garnet is slightly harder than quartz1 and its hardness is said therefore to be #g4 ,delicate tests show that the structure of all crystallized substances is more or less grained1 like that of wood1 and the hardness for the same stone varies in different directions4 ,kyanite is unique in this respect1 since its hardness ranges from #e to #g2 it can therefore be scratched by a knife in some directions1 but not in others4 ,in most substances1 however1 the range is so small as to be quite imperceptible4 ,slight variation is also apparent in the hardness of different specimens of the same species4 ,the diamonds from ,borneo and ,new ,south ,wales are so distinctly harder than those from ,south ,africa and other localities that1 when first discovered1 some difficulty was experienced in cutting them4 ,again1 lapidaries find that while ,ceylon sapphires are harder than rubies1 ,kashmir sapphires are softer4 ,hardness is a character of fundamental importance in a stone intended for ornamental wear1 since upon it depends the durability of the polish and brilliancy4 ,ordinary dust is largely composed of grains of sand1 which is quartz in a minute form1 and a gem-stone should therefore be at least as hard as that4 ,paste imitations are little harder than #e1 and consequently1 as experience shows1 their polish does not survive a few weeks' wear4 ,hardness is1 however1 of little use as a discriminative test except for distinguishing between topaz or harder stone and paste4 ,diamond is so much harder than other stones that it will leave a cut in glass quite different from the scratch of even corundum4 ,paste1 being so soft1 readily yields to the file1 and is thus easily distinguished from genuine stones4 ,in applying the test to a cut stone1 it is best to remove it from its mount and try the effect on the girdle1 because any scratch would be concealed afterwards by the setting4 ,any mark should be rubbed with the finger to assure that it is not due to powder from the scratching agent2 confusion may often be caused in this way when the two substances are of nearly the same hardness4 ,the degrees of hardness of the gem-stones are given in ,table ,,viii at the end of the book4 ,it must not be overlooked that extreme hardness is compatible with cleavability in certain directions intimately connected with the crystalline structure2 the property1 in fact1 characterizes many mineral species of different degrees of hardness4 ,diamond can be split in four directions parallel to the faces of the regular octahedron1 a property utilized by the lapidary for shaping a stone previous to cutting it4 ,topaz cleaves with considerable ease at right angles to the principal crystallographic axis4 ,felspar has two directions of cleavage nearly at right angles to one another4 ,the new gem-stone1 kunzite1 needs cautious handling owing to the facility with which it splits in two directions mutually inclined at about #gj4 ,all stones are more or less brittle1 and will be fractured by a sufficiently violent blow1 but the irregular surface of a fracture cannot be mistaken for the brilliant flat surface given by a cleavage4 ,the cleavage is by no means induced with equal facility in the species mentioned above4 ,a considerable effort is required to split diamond1 but in the case of topaz or kunzite incipient cleavage in the shape of flaws may be started if the stone be merely dropped on to a hard floor4 ,,chapter ,x ,,electrical ,,characters ,the definite orientation of the molecular arrangement of crystallized substances leads in many cases to attributes which vary with the direction and are revealed by the electrical properties4 ,if a tourmaline crystal be heated in a gas or alcohol flame it becomes charged with electricity1 and1 since it is at the same time a bad conductor1 static charges of opposite sign appear at the two ends4 ,topaz shows similar characters1 but in a lesser degree4 ,quartz1 if treated in the same way1 shows charges of opposite sign on different sides1 but the phenomenon may be masked by intimate twinning and consequent overlapping of the contrary areas4 ,the phenomenon may also be seen when the stones are cut4 ,the most convenient method for detecting the existence of the electrical charges is that devised by ,kundt4 ,a powder consisting of a mixture of red lead and sulphur is placed in a bellows arrangement and blown through a sieve at one end on to the stone4 ,owing to the friction the particles become electrified--red lead positively and sulphur negatively--and are attracted by the charges of opposing sign1 which will therefore be betrayed by the colour of the dust at the corresponding spot4 ,the powder must be kept dry2 otherwise a chemical reaction may occur leading to the formation of lead sulphide1 recognizable by its black colour4 ,bcker has suggested as an alternative the use of sulphur1 coloured red with carmine1 the negative element1 and yellow lycopodium1 the positive element4 ,diamond1 topaz1 and tourmaline are powerful enough1 when electrified by friction with a cloth1 to attract fragments of paper1 the electrification being positive4 ,amber develops considerable negative electricity when treated in a similar manner4 ,diamond is translucent to the ,rntgen 7,x7 rays2 glass1 on the other hand1 is opaque to them1 and this test distinguishes brilliants from paste imitations4 ,diamond also1 unlike glass1 phosphoresces under the influence of radium1 a property characterizing also kunzite4 ,it will be seen that the electrical characters1 although of considerable interest to the student1 are1 on account of their limited application and difficulty of test1 of little service for the discrimination of gem-stones4 ,,part ,i--,,section ,b ,,the ,,technology ,,of ,,gem-,,stones ,,chapter ,,xi ,,unit ,,of ,,weight ,the system in use for recording the weights of precious stones is peculiar to jewellery4 ,the unit1 which is known as the carat1 bears no simple relation to any unit that has existed among ,european nations1 and indubitably has been introduced from the ,east4 ,when man in early days sought to record the weights of small objects1 he made use of the most convenient seeds or grains which were easily obtainable and were at the same time nearly uniform in size4 ,in ,europe the smallest unit of weight was the barley grain4 ,similarly in the ,east the seeds of some leguminous tree were selected4 ,those of the locust-tree1 ,ceratonia siliqua1 which is common in the countries bordering the ,mediterranean1 on the average weigh so nearly a carat that they almost certainly formed the original unit4 ,it is1 indeed1 from the ,greek 1 little horn1 which refers to the shape of the pods1 that the word carat is derived4 ,it is one of the eccentricities of the jewellery trade that precision should not have been given to the unit of weight4 ,not only does it vary at most of the trade centres in the world1 but it is not even always constant at each centre4 ,the difference is negligible in the case of single stones of ordinary size1 but becomes a matter of serious importance when large stones1 or parcels of small stones1 are bought and sold1 particularly when the stones are very costly4 ,attempts have been made at various times to secure a uniform standard1 but as yet with only partial success4 ,in #ahga the carat defined as the equivalent of #jbjejj gram was suggested at a meeting of the principal jewellers of ,paris and ,london1 and was eventually accepted in ,paris1 ,new ,york1 ,leipzig1 and ,borneo4 ,it has1 however1 recently been recognized that in view of the gradual spread of the metric system of weights and measures the most satisfactory unit is the metric carat of one-fifth 7#jb7 gram4 ,this has now been constituted the legal carat of ,france and ,belgium1 and no doubt other countries will follow their example4 ,the carat weight obtaining in ,london weighs about #jbjecj gram1 and the approximate equivalents in the gram at other centres are as follows3--,florence #jaigbj1 ,madrid #jbjeci1 ,berlin #jbjedd1 ,amsterdam #jbjegj1 ,lisbon #jbjege1 ,frankfort-on-,main #jbjegg1 ,vienna #jbjfac1 ,venice #jbjgjj1 and ,madras #jbjgce4 ,the gram itself is inconveniently large to serve as a unit for the generality of stones met with in ordinary jewellery4 ,the notation for expressing the sub-multiples of the carat forms another curious eccentricity4 ,fractions are used which are powers of the half3 thus the half1 the half of that1 i4e4 the quarter1 and so on down to the sixty-fourth1 and the weight of a stone is expressed by a series of fractions1 e4g4 #c #a_/fd carats4 ,in the case of diamond a single unreduced fraction to the base #fd is substituted in place of the series of single fractions1 and the weight of a stone is stated thus1 #d-dj_/fd carats4 ,with the introduction of the metric carat the more convenient and rational decimal notation would1 of course1 be simultaneously adopted4 ,illustration3 ,figs4 #cd-ci4--;,exact ,sizes of ,brilliants of various ,weights4 ,figs4 #cd-ci illustrate the exact sizes of diamonds of certain weights1 when cut as brilliants4 ,the sizes of other stones depends upon their specific gravity1 the weight varying as the volume multiplied by the specific gravity4 ,quartz1 for instance1 has a low specific gravity and would be perceptibly larger1 weight for weight2 zircon1 on the other hand1 would be smaller4 ,it has been found more convenient to select a smaller unit in the case of pearls1 namely1 the pearl-grain1 four of which go to the carat4 ,stencil gauges are in use for measuring approximately the weight in carats of diamond brilliants and of pearls1 which in both instances must be unmounted4 ,a more accurate method for determining the weight of diamonds has been devised by ,charles ,moe1 which is applicable to either unmounted or mounted stones4 ,by means of callipers1 which read to three-tenths of a millimetre1 the diameter and the depth of the stone are measured1 and by reference to a table the corresponding weight is found2 allowance is made for the varying fineness of the girdle1 and1 in the case of large stones1 for the variation from a strictly circular section4 ,since this chapter was written the movement in favour of the metric carat has made rapid progress1 and this unit will soon have been adopted as the legal standard all over the world1 even in countries1 such as the ,british ,isles and the ,united ,states1 where the metric system is not in use4 ,the advantage of an international unit is too obvious to need arguing4 ,,chapter ,,xii ,,fashioning ,,of ,,gem-,,stones ,although many of the gem-stones have been endowed by nature with brilliant lustrous faces and display scintillating reflections from their surfaces1 yet their form is never such as to reveal to full perfection the optical qualities upon which their charm depends4 ,moreover1 the natural faces are seldom perfect2 as a rule the stones are broken either through some convulsion of the earth's crust or in course of extraction from the matrix in which they have lain1 or they are roughened by attrition against matter of greater hardness1 or worn by the prolonged action of water1 or etched by solvents4 ,beautiful octahedra of diamond or spinel have been mounted without further embellishment1 but even their appearance might have been much improved at the lapidary's hands4 ,by far the oldest of the existing styles of cutting is the rounded shape known as cabochon1 a ,french word derived from the ,latin cabo1 a head4 ,in the days of the ,roman ,empire the softer stones were often treated in this manner2 such stones were supposed to be beneficial to those suffering from short-sightedness1 the reason no doubt being that transparent stones when cut as a double cabochon formed a convex lens4 ,according to ,pliny1 ,nero had an emerald thus cut1 through which he was accustomed to view the gladiatorial shows4 ,this style of cutting was long a favourite for coloured stones1 such as emerald1 ruby1 sapphire1 and garnet1 but has been abandoned in modern practice except for opaque1 semi-opaque1 and imperfect stones4 ,the crimson garnet1 which was at one time known by the name carbuncle1 was so systematically thus cut that the word has come to signify a red garnet of this form4 ,it was a popular brooch-stone with our grandmothers1 but is no longer in vogue4 ,the ,east still retains a taste for stones cut in the form of beads and drilled through the centre2 the beads are threaded together1 and worn as necklaces4 ,the native lapidaries often improve the colour of pale emeralds by lining the hole with green paint4 ,illustration3 ,,plate ,v ,,jewellery ,,designs ,illustration3 ,,fig4 #dj4--;,double 7,convex7 ,cabochon4 ,illustration3 ,,fig4 #da4--,simple ,cabochon4 ,the cabochon form may be of three different kinds4 ,in the first1 the double cabochon 7,fig4 #dj71 both the upper and the under sides of the stones are curved4 ,the curvature1 however1 need not be the same in each case2 indeed1 it is usually markedly different4 ,moonstones and starstones are generally cut very steep above and shallow underneath4 ,occasionally a ruby or a sapphire is1 when cut in this way1 set with the shallow side above1 because the light that has penetrated into the stone from above is more wholly reflected from a steep surface with consequent increase in the glow of colour from the stone4 ,opals are always cut higher on the exposed side1 but the slope of the surface varies considerably2 they are generally cut steeply when required for mounting in rings4 ,chrysoberyl cat's-eyes are invariably cut with curved bases in order to preserve the weight as great as possible4 ,the double cabochon form with a shallow surface underneath merges into the second kind 7,fig4 #da7 in which the under side is plane1 the form commonly employed for quartz cat's-eyes1 and occasionally also for carbuncles4 ,in this type the plane side is invariably mounted downwards4 ,in the third form 7,fig4 #db7 the curvature of the under surface is reversed1 and the stone is hollowed out into a concave shape4 ,this style is reserved for dark stones1 such as carbuncles1 which1 if cut at all thick1 would show very little colour4 ,a piece of foil is often placed in the hollow in order to increase the reflection of light1 and thus to heighten the colour effect4 ,illustration3 ,,fig4 #db4--;,double 7,concavo-convex7 ,cabochon4 ,in early days it was supposed that the extreme hardness of diamond precluded the possibility of fashioning it1 and up to the fifteenth century all that was done was to remove the gum-like skin which disfigured the ,indian stones and to polish the natural facets4 ,the first notable advance was made in #adge1 when ,louis de ,berquem discovered1 as it is said quite by accident1 that two diamonds if rubbed together ground each other4 ,with confident courage he essayed the new art upon three large stones entrusted to him by ,charles the ,bold1 to the entire satisfaction of his patron4 ,the use of wheels or discs charged with diamond dust soon followed1 but at first the lapidaries evinced their victory over such stubborn material by grinding diamond into divers fantastic shapes1 and failed to realize how much might be done to enhance the intrinsic beauty of the stones by the means now at their disposal4 ,the ,indian lapidaries arrived at the same discovery independently1 and ,tavernier found1 when visiting the country in #affe1 a large number of diamond cutters actively employed4 ,if the stone were perfectly clear1 they contented themselves with polishing the natural facets2 but if it contained flaws or specks1 they covered it with numerous small facets haphazardly placed4 ,the stone was invariably left in almost its original shape1 and no effort was made to improve the symmetry4 ,illustration3 ,,fig4 #dc4--,table ,cut 7top view74 ,illustration3 ,,fig4 #dd4--,table ,cut 7side view74 ,for a long time little further progress was made1 and even nearly a century after ,berquem the only regular patterns known to ,kentmann1 who wrote in #aefb1 were the diamond-point and the diamond-table 7,figs4 #dc-dd74 ,the former consisted of the natural octahedron facets ground to regular shape1 and was long employed for the minute stones which were set in conjunction with large coloured stones in rings4 ,the table represented considerably greater labour4 ,one corner of the regular octahedron was ground down until the artificial facet thus produced was half the width of the stone1 while the opposite corner was slightly ground4 ,still another century elapsed before the introduction of the rose pattern1 which comprised twenty-four triangular facets and a flat base 7,figs4 #de-df71 the stone being nearly hemispherical in shape4 ,this style is said to have been the invention of ,cardinal ,mazarin1 but probably he was the first to have diamonds of any considerable size cut in this form4 ,at the present day only tiny stones are cut as roses4 ,illustration3 ,,fig4 #de4--,rose ,cut 7top view74 ,a few more years passed away1 and at length at the close of the seventeenth century diamond came by its own when ,vincenzio ,peruzzi1 a ,venetian1 introduced the brilliant form of cutting1 and revealed for the first time its amazing #8fire4' ,except for minor changes this form remains to this day the standard style for the shape of diamond1 and the word brilliant is commonly employed to denote diamond cut in this way4 ,so obviously and markedly superior is the style to all others that upon its discovery the owners of large roses had them re-cut as brilliants despite the loss in weight necessitated by the change4 ,illustration3 ,,fig4 #df4--,rose ,cut 7side view74 ,the brilliant form is derived from the old table by increasing the number of facets and slightly altering the angles pertaining to the natural octahedron4 ,in a perfect brilliant 7,figs4 #dg-di7 there are altogether #eh facets1 #cc above and #be below the girdle1 as the edge separating the upper and lower portions of the stone is termed1 which are arranged in the following manner4 ,eight star-facets1 triangular in shape1 immediately surround the large table-facet4 ,next come four large templets or bezels1 quadrilateral in form1 arranged in pairs on opposite sides of the table-facet1 the four quoins or lozenges1 similar in shape1 coming intermediately between them2 in modern practice1 however1 these two sets are identical in shape and size1 and there are consequently eight facets of the same kind instead of two sets of four4 ,the eight cross or skew facets and the eight skill facets1 in both sets the shape being triangular1 form the boundary of the girdle2 modern brilliants usually have instead sixteen facets of the same shape and size4 ,the above #cc facets lie above the girdle and form the crown of the stone4 ,immediately opposite and parallel to the table is the tiny culet4 ,next to the latter come the four large pavilion facets with the four quoins intermediately between them1 both sets being five-sided but nearly quadrilateral in shape2 these again are usually combined into eight facets of the same size4 ,eight cross facets and eight skill facets1 both sets1 like those in the crown1 being triangular in shape1 form the lower side of the girdle2 these also are generally united into a set of sixteen similar facets4 ,these #be facets which lie below the girdle comprise the #8pavilion1' or base of the stone4 ,in a regular stone properly cut a templet is nearly parallel to a pavilion1 and an upper to a lower cross facet4 ,the contour of the girdle is usually circular1 but occasionally assumes less symmetrical shapes1 as for instance in drop-stones or pendeloques1 and the facets are at the same time distorted4 ,the number of facets may with advantage be increased in the case of large stones4 ,an additional set of eight star facets is often placed round the culet1 the total number then being #ff4 ,it may be mentioned that the largest stone cut from the ,cullinan has the exceptional number of #gd facets4 ,illustration3 ,,fig4 #dg4--;,brilliant ,cut 7top view74 ,illustration3 ,,fig4 #dh4--;,brilliant ,cut 7base view74 ,illustration3 ,,fig4 #di4--;,brilliant ,cut 7side view74 ,in order to secure the finest optical effect certain proportions have been found necessary4 ,the depth of the crown must be one-half that of the base1 and therefore one-third the total depth of the stone1 and the width of the table must be slightly less than half that of the stone4 ,the culet should be quite small1 not more in width than one-sixth of the table2 it is1 in fact1 not required at all except to avoid the danger of the point splintering4 ,the girdle should be as thin as is compatible with strength sufficient to prevent chipping in the process of mounting the stone2 if it were left thick1 the rough edge would be visible by reflection at the lower facets1 and would1 especially if at all dirty1 seriously affect the quality of the stone4 ,the shape of the stone is largely determined by the sizes of the templets in the crown and the pavilions in the base as compared with that of the table1 or1 what comes to the same thing1 by the inclinations at which they are cut to that facet4 ,if the table had actually half the width of the stone1 the angle#e between it and a templet would be exactly half a right angle or #de2 it is1 however1 made somewhat smaller1 namely1 about #dj4 ,a pavilion1 being parallel to a templet1 makes a similar angle with the culet4 ,the cross facets are more steeply inclined1 and make an angle of about #de with the table or the culet1 as the case may be4 ,the star facets1 on the other hand1 slant perceptibly less1 and make an angle of only about #bf with the table4 ,a latitude of some #d or #e is possible without seriously affecting the #8fire' of the stone4 ,the object of the disposition of the facets on a brilliant is to assure that all the light that enters the stone1 principally by way of the table1 is wholly reflected from the base and emerges through the crown1 preferably by way of the inclined facets4 ,a brilliant-cut diamond1 if viewed with the table between the observer and the light1 appears quite dark except for the small amount of light escaping through the culet4 ,light should therefore fall on the lower facets at angles greater than the critical angle of total-reflection1 which for diamond is #bd #bf4 ,the pavilions should be inclined properly at double this angle1 or #dh #eb1 to the culet2 but a ray that emerges at a pavilion in the actual arrangement entered the table at nearly grazing incidence1 and the amount of light entering this facet at such acute perspective is negligible4 ,on the other hand1 after reflection at the base light must1 in order to emerge1 fall on the crown at less than the critical angle of total-reflection4 ,in ,fig4 #ej are shown diagrammatically the paths of rays that entered the table in divers ways4 ,the ray emerging again at the table suffers little or no dispersion and is almost white1 but those coming out through the inclined facets are split up into the rainbow effect1 known as #8fire1' for which diamond is so famous4 ,it is in order that so much of the light entering by the table may emerge through the inclined facets of the crown that the pavilions are inclined at not much more than #dj to the culet4 ,it might be suggested that instead of being faceted the stone should be conically shaped1 truncated above and nearly complete below4 ,the result would no doubt be steadier1 but1 on the other hand1 far less pleasing4 ,it is the ever-changing nuance that chiefly attracts the eye2 now a brilliant flash of purest white1 anon a gleam of cerulean blue1 waxing to richest orange and dying in a crimson glow1 all intermingled with the manifold glitter from the surface of the stone4 ,absolute cleanliness is essential if the full beauty of any stone is to be realized1 but this is particularly true of diamond4 ,if the back of the stone be clogged with grease and dirt1 as so often happens in claw-set rings1 light is no longer wholly reflected from the base2 much of it escapes1 and the amount of #8fire' is seriously diminished4 ,illustration3 ,,fig4 #ej4--;,course of the ,rays of ,light passing through a ,brilliant4 ,needless to state1 lapidaries make no careful angular measurements when cutting stones1 but judge of the position of the facets entirely by eye4 ,it sometimes therefore happens that the permissible limits are overstepped1 in which event the stone is dead and may resist all efforts to vivify it short of the heroic course of re-cutting it1 too expensive a treatment in the case of small stones4 ,the factors that govern the properties of a brilliant-cut stone are large colour-dispersion1 high refraction1 and freedom from any trace of intrinsic colour4 ,the only gem-stone that can vie with diamond in these respects is zircon4 ,although it is rare to find a zircon naturally without colour1 yet many kinds are easily deprived of their tint by the application of heat4 ,a brilliant-cut zircon is1 indeed1 far from readily distinguished by eye from diamond1 and has probably often passed as one1 but it may easily be identified by its large double refraction 7cf4 p4 #da7 and inferior hardness4 ,the remaining colourless stones1 such as white sapphire1 topaz1 and quartz 7rock-crystal71 have insufficient refractivity to give total-reflection at the base1 and1 moreover1 they are comparatively deficient in #8fire4' ,illustration3 ,,fig4 #ea4--,step- or ,trap-,cut 7top view74 ,illustration3 ,,fig4 #eb4--,step- or ,trap-,cut 7side view74 ,a popular style of cutting which is much in vogue for coloured stones is the step- or trap-cut1 consisting of a table and a series of facets with parallel horizontal edges 7,figs4 #ea-eb7 above and below the girdle2 in recent jewellery1 however1 the top of the stone is often brilliant-cut4 ,the contour may be oblong1 square1 lozenge1 or heart-shaped1 or have less regular forms4 ,the table is sometimes slightly rounded4 ,since the object of this style is primarily to display the intrinsic colour of the stone and not so much a brilliant play of light from the interior1 no attempt is made to secure total-reflection at the lower facets4 ,the stone therefore varies in depth according to its tint2 if dark1 it is cut shallow1 lest light be wholly absorbed within1 and the stone appear practically opaque1 but if light1 it is cut deep1 in order to secure fullness of tint4 ,much precision in shape and disposition of the facets is not demanded1 and the stones are usually cut in such a way that1 provided the desired effect is obtained1 the weight is kept as great as possible2 we may recall that stones are sold by weight4 ,in considering what will be the optical effect of any particular shape1 regard must be had to the effective colour of the transmitted light4 ,for instance1 although sapphire and ruby belong to the same species and have the same refractive indices1 yet1 since the former transmits mainly blue and the latter red light1 they have for practical purposes appreciably different indices1 and lapidaries find it therefore possible to cut the base of ruby thicker than that of sapphire1 and thus keep the weight greater4 ,it is instructive too what can be done with the most unpromising material by the exercise of a little ingenuity4 ,thus ,ceylon sapphires are often so irregularly coloured that considerable skill is called for in cutting them4 ,a stone may1 for instance1 be almost colourless except for a single spot of blue2 yet1 if the stone be cut steeply and the spot be brought to the base1 the effect will be precisely the same as if the stone were uniformly coloured1 because all the light emerging from the stone has passed through the spot at the base and therefore been tinted blue4 ,the mechanism employed in the fashioning of gem-stones is simple in character1 and comprises merely metal plates or wheels for slitting1 and discs or laps for grinding and polishing the stones1 the former being set vertically and rotated about horizontal spindles1 and the latter set horizontally and rotated about vertical spindles4 ,mechanical power is occasionally used for driving both kinds of apparatus1 but generally1 especially in slitting and in delicate work1 hand-power is preferred4 ,in the ,east native lapidaries make use of vertical wheels 7,plate ,,xiii7 also for grinding and polishing stones1 which explains why native-cut stones never have truly plane facets2 it will be noticed from the picture that a long bow is used to drive the spindle4 ,owing to the unique hardness of diamond it can be fashioned only by the aid of its own powder4 ,the process differs therefore materially from the cutting of the remaining gem-stones1 and will be described separately4 ,indeed1 so different are the two classes of work that firms seldom habitually undertake both4 ,the discovery of the excellent cleavage of diamond enormously reduced the labour of cutting large stones4 ,a stone containing a bad flaw may be split to convenient shape in as many minutes as the days or even weeks required to grind it down4 ,the improvement in the appliances and the provision of ample mechanical power has further accelerated the process and reduced the cost4 ,two years were occupied in cutting the diamond known as the ,pitt or ,regent1 whereas in only six months the colossal ,cullinan was shaped into two large and over a hundred smaller stones with far less loss of material4 ,although the brilliant form was derived from the regular octahedron1 it by no means follows that1 because diamond can be cleaved to the latter form1 such is the initial step in fashioning the rough mass4 ,the aim of the lapidary is to cut the largest possible stone from the given piece of rough1 and the finished brilliant usually bears no relation whatever to the natural octahedron4 ,the cleavage is utilized only to free the rough of an awkward and useless excrescence1 or of flaws4 ,although the octahedron is one of the common forms in which diamond is found1 it is rarely regular1 and oftener than not one of the larger faces is made the table4 ,the old method1 which is still in use1 for roughly fashioning diamonds is that known as bruting1 from the ,french word1 brutage1 for the process1 or as shaping4 ,two stones of about the same size are selected1 and are firmly attached by means of a hard cement to the ends of two holders1 which are held one in each hand1 and rubbed hard1 one against the other1 until surfaces of the requisite size are developed on each stone4 ,during the process the stones are held over a small box1 which catches the precious powder4 ,a fine sieve at the bottom of the box allows the powder to fall through into a tray underneath1 but holds back anything larger4 ,by means of two vertical pins placed one on each side of the box the holders are retained more easily in the desired position1 and the work is thrown mainly on the thumbs4 ,this work continued day after day has a very disfiguring effect upon the hands despite the thick gloves that are worn to protect them2 the skin of the thumbs grows hard and horny1 and the first and second fingers become swollen and distorted4 ,when the surfaces have thus been formed1 the stone is handed to the polisher1 who works them into the correct shape and afterwards polishes them1 the stone passing backwards and forwards several times between the cutter and the polisher4 ,the table1 four templets1 culet and four pavilions are first formed and polished1 so that the table has a square shape4 ,next the quoins are developed and polished1 and finally the small facets are polished on1 not being shaped first4 ,in modern practice the process of bruting has been modified in some cases by the introduction of machinery1 and the facets are ground on1 with considerable improvement in the regularity of their size and disposition1 and reduction in the amount of polishing required4 ,moreover1 to obviate the loss of material resulting from continued grinding1 large stones are first sliced by means of rapidly-revolving copper wheels charged with diamond powder4 ,the laps used for polishing diamonds are made of a particular kind of soft iron1 which is found to surpass any other metal in retaining the diamond powder4 ,they are rotated at a high rate of speed1 which is about #bjjj to #bejj revolutions a minute1 and the heat developed by the friction at this speed is too great for a cement to be used2 a solder or fusible alloy1 composed of one part tin to three parts lead1 therefore takes its place4 ,the solder is held in a hollow cup of brass which is from its shape called a #8dop1' an old ,dutch word meaning shell4 ,its external diameter is ordinarily about #a in4 7#d cm471 but larger dops are1 of course1 used for large stones4 ,a stout copper stalk is attached to the bottom of the dop2 it is visible in the view of the dop shown at e on ,plate ,,vi1 and two slabs of solder are seen lying in front of the dop4 ,the dop containing the solder is placed in the midst of a non-luminous flame and heated until the solder softens1 when it is removed by means of the small tongs1 c1 and placed upright on a stand such as that shown at a4 ,the long tongs1 d1 are used for shaping the solder into a cone at the apex of which the diamond is placed4 ,the solder is worked well over the stone so that only the part to undergo polishing is exposed4 ,a diamond in position is shown at f4 ,the top of the stand is saucer-shaped to catch the stone should it accidentally fall off the dop1 and to prevent pieces of solder falling on the hand4 ,while still hot1 the dop with the diamond in position on the solder is plunged into cold water in order to cool it4 ,the fact that the stone withstands this drastic treatment is eloquent testimony to its good thermal conductivity2 other gem-stones would promptly split into fragments4 ,it may be remarked that so high is the temperature at which diamond burns that it may be placed in the gas flame without any fear of untoward results4 ,the dop is now ready for attachment to an arm such as that shown at b2 the stalk of the dop is placed in a groove running across the split end of the arm1 and is gripped tight by means of a screw worked by the nut which is visible in the picture4 ,illustration3 ,,plate ,,vi ,,appliances ,,used ,,for ,,polishing ,,diamonds4 ,illustration3 ,,plate ,,vii ,,polishing ,,diamonds ,four such arms1 each with a dop1 are used with the polishing lap 7,plate ,,vii71 and each stands on two square legs on the bench4 ,pins1 p1 in pairs are fixed to the bench to prevent the arms being carried round by the friction2 one near the lap holds the arm not far from the dop1 and the other engages in a strong metal tongue1 which is best seen at the end of the arm b on ,plate ,,vi4 ,though the arm1 which is made of iron1 is heavy1 yet for polishing purposes it is insufficient1 and additional lead weights are laid on the top of it1 as in the case of the arm at the back on ,plate ,,vii4 ,the copper stalk is strong1 yet flexible1 and can be bent to suit the position of the facet to be polished2 on ,plate ,,vii the dops a and b are upright1 but the other two are inclined4 ,in addition to the powder resulting from bruting1 boart1 i4e4 diamonds useless for cutting1 are crushed up to supply polishing material1 and a little olive oil is used as a lubricant4 ,owing to the friction so much heat is developed that even the solder would soften after a time1 and therefore1 as a precaution1 the dop is from time to time cooled by immersion in water4 ,the stone has constantly to be re-set1 about six being the maximum even of the tiny facets near the girdle that can be dealt with by varying the inclination of the dop4 ,as the work approaches completion the stone is frequently inspected1 lest the polishing be carried too far for the development of the proper amount of #8fire4' ,when finished1 the stones are boiled in sulphuric acid to remove all traces of oil and dirt4 ,the whole operation is evidently rough and ready in the extreme2 but such amazing skill do the lapidaries acquire1 that even the most careful inspection by eye alone would scarce detect any want of proper symmetry in a well-cut stone4 ,the fashioning of coloured stones1 as all the gem-stones apart from diamond are termed in the jewellery trade1 is on account of their inferior hardness a far less tedious operation4 ,they are easily slit1 for which purpose a vertical wheel 7,plate ,,viii7 made of soft iron is used2 it is charged with diamond dust and lubricated with oil1 generally paraffin4 ,when slit to the desired size1 the stone is attached to a conveniently shaped holder by means of a cement1 the consistency of which varies with the hardness of the stone4 ,it is set in the cement in such a way that the plane desired for the table facet is at right angles to the length of the holder1 and the whole of the upper part or crown is finished before the stone is removed from the cement4 ,the lower half or base is treated in a similar manner4 ,thus in the process of grinding and polishing the stone is only once re-set2 as was stated above1 diamond demands very different treatment4 ,again1 all coloured stones are ground down without any intermediate operation corresponding to bruting4 ,the holder is merely held in the hand1 but to maintain its position more exactly its other end1 which is pointed1 is inserted in one of the holes that are pierced at intervals in a vertical spindle placed at a convenient distance from the lap 7,plate ,,viii71 which one depending upon the inclination of the facet to be formed4 ,for hard stones1 such as ruby and sapphire1 diamond powder is generally used as the abrasive agent1 while for the softer stones emery1 the impure corundum1 is selected2 in recent years the artificially prepared carborundum1 silicide of carbon corresponding to the formula ,c,si1 which is harder than corundum1 has come into vogue for grinding purposes1 but it is unfortunately useless for slitting1 because it refuses to cling to the wheel4 ,to efface the scratches left by the abrasive agent and to impart a brilliant polish to the facets1 material of less hardness1 such as putty-powder1 pumice1 or rouge1 is employed2 in all cases the lubricant is water4 ,the grinding laps are made of copper1 gun-metal1 or lead2 and pewter or wooden laps1 the latter sometimes faced with cloth or leather1 are used for polishing4 ,as a general rule1 the harder the stone the greater the speed of the lap4 ,illustration3 ,,plate ,,viii ,,slitting ,,coloured ,,stones ,,polishing ,,coloured ,,stones ,illustration3 ,,plate ,,ix ,,faceting ,,machine ,as in the case of diamond1 the lapidary judges of the position of the facet entirely by eye and touch1 but a skilled workman can develop a facet very close to the theoretical position4 ,during recent years various devices have been invented to enable him to do his work with greater facility4 ,a machine of this kind is illustrated on ,plate ,,ix4 ,the stone is attached by means of cement to the blunt end1 d1 of the holder1 b1 which is of the customary kind1 while the other end is inserted in a hole in a wooden piece1 a1 which is adjustable in height by means of the screw above it4 ,the azimuthal positions of the facets are arranged by means of the octagonal collar1 c1 the sides of which are held successively in turn against the guide1 e4 ,the stand itself is clamped to the bench4 ,the machine is1 however1 little used except for cheap stones1 because it is too accurate and leads to waste of material4 ,stones are sold by weight1 and so long as the eye is satisfied1 no attempt is made to attain to absolute symmetry of shape4 ,the pictures on ,plates ,x-,,xiii illustrate lapidaries' workshops in various parts of the world4 ,the first two show an office and a workshop situated in ,hatton ,garden1 ,london2 in the former certain of the staff are selecting from the parcels stones suitable for cutting4 ,the third depicts a more primitive establishment at ,ekaterinburg in the ,urals4 ,the fourth shows a typical ,french family--pre1 mre1 et fils--in the ,jura district1 all busily engaged2 on the table will be noticed a faceting machine of the kind described above4 ,in the fifth picture a native lapidary in ,calcutta is seen at work with the driving bow in his right1 and the stone in his left1 hand4 ,a curious difference exists in the systems of charging for cutting diamonds and coloured stones4 ,the cost of cutting the latter is reckoned by the weight of the finished stone1 the rate varying from #as4 to #hs4 a carat according to the character of the stone and the difficulty of the work2 while in the case of diamonds1 on the other hand1 the weight of the rough material determines the cost1 the rate being about #ajs4 to #djs4 a carat according to the size1 which on the average is equivalent to about #cjs4 to #abjs4 a carat calculated on the weight of the finished stone4 ,the reason of the distinction is obviously because the proper proportions in a brilliant-cut diamond must be maintained1 whatever be the loss in weight involved2 in coloured stones the shape is not of such primary importance4 ,illustration3 ,,plate ,x ,,lapidary',s ,,workshop ,,and ,,office ,,in ,,england ,illustration3 ,,plate ,,xi ,,lapidary',s ,,workshop ,,in ,,russia ,when finished1 the stone finds its way with others akin to it to the manufacturing jeweller's establishment1 where it is handed to the setter1 who mounts it in a ring1 necklace1 brooch1 or whatever article of jewellery it is intended for4 ,the metal used in the groundwork of the setting is generally gold1 but platinum is also employed where an unobtrusive and untarnishable metal is demanded1 and silver finds a place in cheaper jewellery1 although it is seriously handicapped by its susceptibility to the blackening influence of the sulphurous fumes present in the smoke-laden atmosphere of towns4 ,the stone may be either embedded in the metal or held by claws4 ,the former is by far the safer1 but the latter the more elegant1 and it has the advantage of exposing the stone jour1 to use the ,french jewellers' expression1 so that its genuineness is more evidently testified4 ,it is very important that the claw setting be periodically examined1 lest the owner one day experience the mortification of finding that a valuable stone has dropped out2 gold1 owing to its softness1 wears away in course of time4 ,up to quite recent years modern jewellery was justly open to the criticism that it was lacking in variety1 that little attempt was made to secure harmonious association in either the colour or the lustre of the gem-stones1 and that the glitter of the gold mount was frequently far too obtrusive4 ,gold consorts admirably with the rich glow of ruby1 but is quite unsuited to the gleaming fire of a brilliant4 ,where the metal is present merely for the mechanical purpose of holding the stones in position1 it should be made as little noticeable as possible4 ,the artistic treatment of jewellery is1 however1 receiving now adequate attention in the best ,paris and ,london houses4 ,some recent designs are illustrated on ,plates ,,iv and ,v4 ,illustration3 ,,plate ,,xii ,,french ,,family ,,cutting ,,stones ,illustration3 ,,plate ,,xiii ,,indian ,,lapidary ,,chapter ,,xiii ,,nomenclature ,,of ,,precious ,,stones ,the names in popular use for the principal gem-stones may be traced back to very early times1 and1 since they were applied long before the determinative study of minerals had become a science1 their significance has varied at different dates1 and is even now far from precise4 ,no ambiguity or confusion could arise if jewellers made use of the scientific names for the species1 but most of them are unknown or at least unfamiliar to those unversed in mineralogy1 and to banish old-established names is undesirable1 even if the task were not hopeless4 ,the name selected for a gem-stone may have a very important bearing on its fortunes4 ,when the love-sick ,juliet queried #8,what's in a name8' her mind was wandering far from jewels2 for them a name is everything4 ,the beautiful red stones that accompany the diamond in ,south ,africa were almost a drug in the market under their proper title--garnet1 but command a ready sale under the misnomer #8,cape-ruby4' ,to many minds there is a subtle satisfaction in the possession of a stone which is assumed to be a sort of ruby that would be destroyed by the knowledge that the stone really belonged to the cinderella species of gem-stones--the despised garnet4 ,for similar reasons it was deemed advisable to offer the lustrous green garnet found some thirty and odd years ago in the ,ural ,mountains as #8olivine1' not a happy choice since their colour is grass- rather than olive-green1 apart from the fact that the term is in general use in science for the species known in jewellery as peridot4 ,the names employed in jewellery are largely based upon the colour1 the least reliable from a determinative point of view of all the physical characters of gem-stones4 ,qualifying terms are employed to distinguish stones of obviously different hardness4 #8,oriental' distinguishes varieties of corundum1 but does not imply that they necessarily came from the ,east2 the finest gem-stones originally reached ,europe by that road1 and the hardest coloured stones consequently received that term of distinction4 ,nearly all red stones are grouped under the name ruby1 which is derived from a ,latin word1 ruber1 meaning red1 or under other names adapted from it1 such as rubellite1 rubicelle4 ,it is properly applied to red corundum2 #8balas' ruby is spinel1 which is associated with the true ruby at the ,burma mines and is similar in appearance to it when cut1 and #8,cape' ruby1 is1 as has been stated above1 a garnet from ,south ,africa4 ,rubellite is the lovely rose-pink tourmaline1 fine examples of which have recently been discovered in ,california1 and rubicelle is a less pronouncedly red spinel4 ,sapphire is by far the oldest and one of the most interesting of the words used in the language of jewels4 ,it occurs in ,hebrew and ,persian1 ancient tongues1 and means blue4 ,it was apparently employed for lapis lazuli or similar substance1 but was transferred to the blue corundum upon the discovery of this splendid stone4 ,oblivious of the real meaning of the word1 jewellers apply it in a quasi-generic sense to all the varieties of corundum with the exception of the red ruby1 and give vent to such incongruous expressions as #8white sapphire1' #8yellow sapphire'2 it is true such stones often contain traces of blue colour1 but that is not the reason of the terms4 #8,brazilian' sapphire is blue tourmaline1 a somewhat rare tint for this species4 ,the curious history of the word topaz will be found below in the chapter dealing with the species of that name4 ,it has always denoted a yellow stone1 and at the present day is applied by jewellers indiscriminately to the true topaz and citrine1 the yellow quartz1 the former1 however1 being sometimes distinguished by the prefix #8,brazilian4' #8,oriental' topaz is corundum1 and #8occidental' topaz is a term occasionally employed for the yellow quartz4 ,emerald1 which means green1 was first used for chrysocolla1 an opaque greenish stone 7p4 #bhh71 but was afterwards applied to the priceless green variety of beryl1 for which it is still retained4 #8,oriental' emerald is corundum1 #8,brazilian' emerald in the eighteenth century was a common term for the green tourmaline recently introduced to ,europe1 and #8,uralian' emerald has been tentatively suggested for the green garnet more usually known as #8olivine4' ,amethyst is properly the violet quartz1 but with the prefix #8oriental' it is also applied to violet corundum1 though some jewellers use it for the brilliant quartz1 with purple and white sectors1 from ,siberia4 ,almandine1 which is derived from the name of an ,eastern mart for precious stones1 has come to signify a stone of columbine-red hue1 principally garnet1 but with suitable qualification corundum and spinel also4 ,the nomenclature of jewellery tends to suggest relations between the gem-stones for which there is no real foundation1 and to obscure the essential identity1 except from the point of view of colour1 of sapphire and ruby1 emerald and aquamarine1 cairngorm and amethyst4 ,,chapter ,,xiv ,,manufactured ,,stones ,the initial step in the examination of a crystallized substance is to determine its physical characters and to resolve it by chemical analysis into its component elements2 the final1 and by far the hardest1 step is to build it up or synthetically prepare it from its constituents4 ,unknown to the world at large1 work of the latter kind has long been going on within the walls of laboratories1 and as the advance in knowledge placed in the hands of experimenters weapons more and more comparable with those wielded by nature1 their efforts have been increasingly successful4 ,so stupendous1 however1 are the powers of nature that the possibility of reproducing1 by human agency1 the treasured stones which are extracted from the earth in various parts of the globe at the cost of infinite toil and labour has always been derided by those ignorant of what had already been accomplished4 ,great1 therefore1 was the consternation and the turmoil when concrete evidence that could not be gainsaid showed that man's restless efforts to bridle nature to his will were not in vain1 and congresses of all the high-priests of jewellery were hastily convened to ban such unrighteous products1 with what ultimate success remains to be seen4 ,crystallization may be caused in four different ways1 of which the second alone has as yet yielded stones large enough to be cut-- #a4 ,by the separation of the substance from a saturated solution4 ,in nature the solvent may not be merely hot water1 or water charged with an acid1 but molten rock1 and the temperature and the pressure may be excessively high4 #b4 ,by the solidification of the liquefied substance upon cooling4 ,ice is a familiar example of this type4 #c4 ,by the sublimation of the vapour of the substance1 which means the direct passage from the vapour to the solid state without traversing the usually intervening liquid state4 ,it is usually the most difficult of attainment of the four methods2 the most familiar instance is snow4 #d4 ,by the precipitation of the substance from a solution when set free by chemical action4 ,other things being equal1 the simpler the composition the greater is the ease with which a substance may be expected to be formed2 for1 instead of one complex substance1 two or more different substances may evolve1 unless the conditions are nicely arranged4 ,attempts1 for instance1 to produce beryl might result instead in a mixture of chrysoberyl1 phenakite1 and quartz4 ,by far the simplest in composition of all the precious stones is diamond1 which is pure crystallized carbon2 but its manufacture is attended by well-nigh insuperable difficulties4 ,if carbon be heated in air1 it burns at a temperature well below its melting point2 moreover1 unless an enormously high pressure is simultaneously applied1 the product is the other form of crystallized carbon1 namely1 the comparatively worthless graphite4 ,moissan's interesting course of experiments were in some degree successful1 but the tiny diamonds were worthless as jewels1 and the expense involved in their manufacture was out of all proportion to any possible commercial value they might have4 ,next to diamond the simplest substances among precious stones are quartz 7crystallized silica7 and corundum 7crystallized alumina74 ,the crystallization of silica has been effected in several ways1 but the value in jewellery of quartz1 even of the violet variety1 amethyst1 is not such as to warrant its manufacture on a commercial scale4 ,corundum1 on the other hand1 is held in high esteem2 rubies and sapphires1 of good colour and free from flaws1 have always commanded good prices4 ,the question of their production by artificial means has therefore more than academic interest4 ,ever since the year #ahcg1 when ,gaudin produced a few tiny flakes1 ,french experimenters have steadily prosecuted their researches in the crystallization of corundum4 ,frmy and ,feil1 in #ahgg1 were the first to meet with much success4 ,a portion of one of their crucibles lined with glistening ruby flakes is exhibited in the ,british ,museum 7,natural ,history74 ,illustration3 ,,fig4 #ec4--,verneuil's ,inverted ,blowpipe4 ,in #ahhe the jewellery market was completely taken by surprise by the appearance of red stones1 emanating1 so it is alleged1 from ,geneva2 having the physical characters of genuine rubies1 they were accepted as1 and commanded the prices of1 the natural stones4 ,it was eventually discovered that they had resulted from the fusion of a number of fragments of natural rubies in the oxy-hydrogen flame4 ,the original colour was driven off at that high temperature1 but was revived by the previous addition of a little bichromate of potassium4 ,owing to the inequalities of growth1 the cracks due to rapid cooling1 the inclusion of air-bubbles1 often so numerous as to cause a cloudy appearance1 and1 above all1 the unnatural colour1 these reconstructed stones1 as they are termed1 were far from satisfactory1 but yet they marked such an advance on anything that had been accomplished before that for some time no suspicion was aroused as to their being other than natural stones4 ,a notable advance in the synthesis of corundum1 particularly of ruby1 was made in #aijd1 when ,verneuil1 who had served his apprenticeship to science under the guidance of ,frmy1 invented his ingenious inverted form of blowpipe 7,fig4 #ec71 which enabled him to overcome the difficulties that had baffled earlier investigators1 and to manufacture rubies vying in appearance after cutting with the best of nature's productions4 ,the blowpipe consisted of two tubes1 of which the upper1 ,e1 wide above1 was constricted below1 and passing down the centre of the lower1 ,f1 terminated just above the orifice of the latter in a fine nozzle4 ,oxygen was admitted at ,c through the plate covering the upper end of the tube1 ,e4 ,a rod1 which passed through a rubber collar in the same plate1 supported inside the tube1 ,e1 a vessel1 ,d1 and at the upper end terminated in a small plate1 on which was fixed a disc1 ,b4 ,the hammer1 ,a1 when lifted by the action of an electromagnet and released1 fell by gravity and struck the disc4 ,the latter could be turned about a horizontal axis placed eccentrically1 so that the height through which the hammer fell and the consequent force of the blow could be regulated4 ,the rubber collar1 which was perfectly gas-tight1 held the rod securely1 but allowed the shocks to be transmitted to the vessel1 ,d1 an arrangement of guides maintaining the slight motion of the vessel strictly vertical4 ,this vessel1 which carried the alumina powder used in the manufacture of the stone1 had as its base a cylindrical sieve of fine mesh4 ,the succession of rapid taps of the hammer caused a regular feed of powder down the tube1 the amount being regulated by varying the height through which the hammer fell4 ,hydrogen or coal-gas was admitted at ,g into the outer tube1 ,f1 and in the usual way met the oxygen just above the orifice1 ,l4 ,to exclude irregular draughts1 the flame was surrounded by a screen1 ,m1 which was provided with a mica window1 and a water-jacket1 ,k1 protected the upper part of the apparatus from excessive heating4 ,illustration3 ,,fig4 #ed4--#8;,boule1' or ,pear-shaped ,drop4 ,the alumina was precipitated from a solution of pure ammonia--alum1 7,,nh#d7b,,so#d4;,al#b7,,so#d7c4bd;,h#b, o1 in distilled water by the addition of pure ammonia1 sufficient chrome-alum also being dissolved with the ammonia-alum to furnish about #b per cent4 of chromic oxide in the resulting stone4 ,the powder1 carefully prepared and purified1 was placed1 as has been stated above1 in the vessel1 ,d1 and on reaching the flame at the orifice it melted1 and fell as a liquid drop1 ,n1 upon the pedestal1 ,p1 which was formed of previously fused alumina4 ,this pedestal was attached by a platinum sleeve to an iron rod1 ,q1 which was provided with the necessary screw adjustments1 ,r and ,s1 for centring and lowering it as the drop grew in size4 ,great care was exercised to free the powder from any trace of potassium1 which1 if present1 imparted a brownish tinge to the stone4 ,the pressure of the oxygen1 low initially both to prevent the pedestal from melting1 and to keep the area of the drop in contact with the pedestal as small as possible1 because otherwise flaws tended to start on cooling1 was gradually increased until the flame reached the critical temperature which kept the top of the drop melted1 but not boiling4 ,the supply of powder was at the same time carefully proportioned to the pressure4 ,the pedestal1 ,p1 was from time to time lowered1 and the drop grew in the shape of a pear 7,fig4 #ed71 the apex of which was downwards and adhered to the pedestal by a narrow stalk4 ,as soon as the drop reached the maximum size possible with the size of the flame1 the gases were sharply and simultaneously cut off4 ,after ten minutes or so the drop was lowered from the chamber1 ,m1 by the screw1 ,s1 and when quite cold was removed from the pedestal4 ,very few changes have been made in the method when adapted to commercial use4 ,coal-gas has1 however1 entirely replaced the costly hydrogen1 and the hammer is operated by a cam instead of an electromagnet1 while1 as may be seen from the view of a gem-stone factory 7,plate ,,xiv71 a number of blowpipes are placed in line so that their cams are worked by the same shaft1 a4 ,the fire-clay screen1 b1 surrounding the flame is for convenience of removal divided into halves longitudinally1 and a small hole is left in front for viewing the stone during growth1 a red glass screen1 c1 being provided in front to protect the eyes from the intense glare4 ,half the fire-clay screen of the blowpipe in the centre of the ,plate has been removed to show the arrangement of the interior4 ,the centring and the raising and lowering apparatus1 d1 have been modified4 ,the process is so simple that one man can attend to a dozen or so of these machines1 and it takes only one hour to grow a drop large enough to be cut into a ten-carat stone4 ,illustration3 ,,plate ,,xiv ,,blowpipe ,,used ,,for ,,the ,,manufacture ,,of ,,rubies ,,and ,,sapphires ,the drops1 unless the finished stone is required to have a similar pear shape1 are divided longitudinally through the central core into halves1 which in both shape and orientation are admirably suited to the purposes of cutting2 as a general rule1 the drop splits during cooling into the desired direction of its own accord4 ,illustration3 ,,fig4 #ee4--;,bubbles and ,curved ,stri in ,manufactured ,ruby4 ,each drop is a single crystalline individual1 and not1 as might have been anticipated1 an alumina glass or an irregular aggregation of crystalline fragments1 and1 if the drop has cooled properly1 the crystallographic axis is parallel to the core of the pear4 ,the cut stone will therefore have not only the density and hardness1 but also all the optical characters--refractivity1 double refraction1 dichroism1 etc4--pertaining to the natural species1 and will obey precisely the same tests with the refractometer and the dichroscope4 ,were it not for certain imperfections it would be impossible to distinguish between the stones formed in ,nature's vast workshop and those produced within the confines of a laboratory4 ,the artificial stones1 however1 are rarely1 if ever1 free from minute air-bubbles 7,fig4 #ee71 which can easily be seen with an ordinary lens4 ,their spherical shape differentiates them from the plane-sided cavities not infrequently visible in a natural stone 7,fig4 #ef74 ,moreover1 the colouring matter varies slightly1 but imperceptibly1 in successive shells1 and consequently in the finished stone a careful eye can discern the curved striations 7,fig4 #ee7 corresponding in shape to the original shell4 ,in a natural stone1 on the other hand1 although zones of different colours or varying shades are not uncommon1 the resulting striations are straight 7,fig4 #ef71 corresponding to the plane faces of the original crystal form4 ,by sacrificing material it might be possible to cut a small stone free from bubbles1 but the curved striations would always be present to betray its origin4 ,illustration3 ,,fig4 #ef4--,markings in ,natural ,ruby4 ,the success that attended the manufacture of ruby encouraged efforts to impart other tints to crystallized alumina4 ,by reducing the percentage amount of chromic oxide1 pink stones were turned out1 in colour not unlike those ,brazilian topazes1 the original hue of which has been altered by the application of heat4 ,these artificial stones have therefore been called #8scientific topaz'2 of course1 quite wrongly1 since topaz1 which is properly a fluo-silicate of aluminium1 is quite a different substance4 ,early attempts made to obtain the exquisite blue tint of the true sapphire were frustrated by an unexpected difficulty4 ,the colouring matter1 cobalt oxide1 was not diffused evenly through the drop1 but was huddled together in splotches1 and it was found necessary to add a considerable amount of magnesia as a flux before a uniform distribution of colour could be secured4 ,it was then discovered that1 despite the colour1 the stones had the physical characters1 not of sapphire1 but of the species closely allied to it1 namely1 spinel1 aluminate of magnesium4 ,by an unsurpassable effort of nomenclature these blue stones were given the extraordinary name of #8,hope sapphire1' from fanciful analogy with the famous blue diamond which was once the pride of the ,hope collection4 ,a blue spinel is occasionally found in nature1 but the actual tint is somewhat different4 ,these manufactured stones have the disadvantage of turning purple in artificial light4 ,by substituting lime for magnesia as a flux1 ,paris1 a pupil of ,verneuil's1 produced blue stones which were not affected to the same extent4 ,the difficulty was at length overcome at the close of #aiji1 when ,verneuil1 by employing as tinctorial agents #je per cent4 of titanium oxide and #ae per cent4 of magnetic iron oxide1 succeeded in producing blue corundum2 it1 however1 had not quite the tint of sapphire4 ,stones subsequently manufactured1 which were better in colour1 contained about #jab per cent4 of titanium oxide1 but no iron at all4 ,by the addition to the alumina of a little nickel oxide and vanadium oxide respectively1 yellow and yellowish green corundums have been obtained4 ,the latter have in artificial light a distinctly reddish hue1 and have therefore been termed #8scientific alexandrite'2 of course1 quite incorrectly1 since the true alexandrite is a variety of chrysoberyl1 aluminate of beryllium1 a very different substance4 ,if no colouring matter at all be added and the alum be free from potash1 colourless stones or white sapphires are formed1 which pass under the name #8scientific brilliant4' ,it is scarcely necessary to remark that they are quite distinct from the true brilliant1 diamond4 ,the high prices commanded by emeralds1 and the comparative success that attended the reconstruction of ruby from fragments of natural stones1 suggested that equal success might follow from a similar process with powdered beryl1 chromic oxide being used as the colouring agent4 ,the resulting stones are1 indeed1 a fair imitation1 being even provided with flaws1 but they are a beryl glass with lower specific gravity and refractivity than the true beryl1 and are wrongly termed #8scientific emerald4' ,moreover1 recently most of the stones so named on the market are merely green paste4 ,it is unfortunate that the real success which has been achieved in the manufacture of ruby and sapphire should be obscured by the ill-founded claims tacitly asserted in other cases4 ,at the time the manufactured ruby was a novelty it fetched as much as #f a carat1 but as soon as it was discovered that it could easily be differentiated from the natural stone1 a collapse took place1 and the price fell abruptly to #cjs41 and eventually to #es4 and even #as4 a carat4 ,the sapphires run slightly higher1 from #bs4 to #gs4 a carat4 ,the prices of the natural stones1 which at first had fallen1 have now risen to almost their former level4 ,the extreme disparity at present obtaining between the prices of the artificial and the natural ruby renders the fraudulent substitution of the one for the other a great temptation1 and it behoves purchasers to beware where and from whom they buy1 and to be suspicious of apparently remarkable bargains1 especially at places like ,colombo and ,singapore where tourists abound4 ,it is no secret that some thousands of carats of manufactured rubies are shipped annually to the ,east4 ,caveat emptor4 ,,chapter ,,xv ,,imitation ,,stones ,the beryl glass mentioned in the previous chapter marks the transition stage between manufactured stones which in all essential characters are identical with those found in nature1 and artificial stones which resemble the corresponding natural stone in outward appearance only4 ,in a sense both sorts may be styled artificial1 but it would be misleading to confound them under the same appellation4 ,common paste1#f which is met with in drapery goods and cheap ornaments in general--hat-pins1 buckles1 and so forth--is composed of ordinary crown-glass or flint-glass1 the refractive indices being about #aec and #afc respectively4 ,the finest quality1 which is used for imitations of brilliants1 is called #8strass4' ,it is a dense lead flint-glass of high refraction and strong colour-dispersion1 consisting of #chb per cent4 of silica1 #ecc red lead 7oxide of lead71 and #gh potassium carbonate1 with small quantities of soda1 alumina1 and other substances4 ,how admirable these imitations may be1 a study of the windows of a shop devoted to such things will show4 ,unfortunately the addition of lead1 which is necessary for imparting the requisite refraction and #8fire' to the strass1 renders the stones exceedingly soft4 ,all glass yields to the file1 but strass stones are scratched even by ordinary window-glass4 ,if worn in such a way that they are rubbed1 they speedily lose the brilliance of their polish1 and1 moreover1 they are susceptible to attack by the sulphurous fumes present in the smoky air of towns1 and turn after a time a dirty brown in hue4 ,when coloured stones are to be imitated1 small quantities of a suitable metallic oxide are fused with the glass2 cobalt gives rise to a royal-blue tint1 chromium a ruby red1 and manganese a violet4 ,common paste is not highly refractive enough to give satisfactory results when cut as a brilliant1 and the bases are therefore often coated with quicksilver1 or1 in the case of old jewellery1 covered with foil in the setting1 in order to secure more complete reflection from the interior4 ,the fashioning of these imitation stones is easy and cheap4 ,being moulded1 they do not require cutting1 and the polishing of the facets thus formed is soon done on account of the softness of the stones4 ,a test with a file readily differentiates paste stones from the natural stones they pretend to be4 ,being necessarily singly refractive1 they are1 of course1 lacking in dichroism1 and their refractivity seldom accords even approximately with that of the corresponding natural stone4 ,in order to meet the test for hardness the doublet was devised4 ,such a stone is composed of two parts--the crown consisting of colourless quartz or other inexpensive real and hard stone1 and the base being made up of coloured glass4 ,when the imitation1 say of a sapphire1 is intended to be more exact1 the crown is made of a real sapphire1 but one deficient in colour1 the requisite tint being obtained from the paste forming the under part of the doublet4 ,in case the base should also be tested for hardness the triplet has been devised4 ,in this the base is made of a real stone also1 and the coloured paste is confined to the girdle section1 where it is hidden by the setting4 ,sapphires and emeralds of indifferent colour are sometimes slit across the girdle2 the interior surfaces are polished1 and colouring matter is introduced with the cement1 generally ,canada balsam1 which is used to re-unite the two portions of the stone together4 ,all such imitations may be detected by placing the stone in oil1 when the surfaces separating the portions of the composite stone will be visible1 or the binding cement may be dissolved by immersing the stone1 if unmounted1 in boiling water1 or in alcohol or chloroform1 when the stone will fall to pieces4 ,the glass imitations of pearls1 which have become very common in recent years1 may1 apart from their inferior iridescence1 be detected by their greater hardness1 or by the apparent doubling of1 say1 a spot of ink placed on the surface1 owing to reflection from the inner surface of the glass shell4 ,they are made of small hollow spheres formed by blowing4 ,next to the glass comes a lining of parchment size1 and next the under lining1 which is the most important part of the imitation1 consisting of a preparation of fish scales called ,essence d',orient1 ,when the lining is dry1 the globe is filled with hot wax to impart the necessary solidity4 ,in cheap imitations the glass balls are not lined at all1 but merely heated with hydrochloric acid to give an iridescence to the surface2 sometimes they are coated with wax1 which can be scraped off with a knife4 ,,part ,,ii--,,section ,a ,,precious ,,stones ,,chapter ,,xvi ,,diamond ,diamond has held pride of place as chief of precious stones ever since the discovery of the form of cutting known as the #8brilliant' revealed to full perfection its amazing qualities2 and justly so1 since it combines in itself extreme hardness1 high refraction1 large colour-dispersion1 and brilliant lustre4 ,a rough diamond1 especially from river gravels1 has often a peculiar greasy appearance1 and is no more attractive to the eye than a piece of washing-soda4 ,it is therefore easy to understand why the ,persians in the thirteenth century placed the pearl1 ruby1 emerald1 and even peridot before it1 and writers in the ,middle ,ages frequently esteemed it below emerald and ruby4 ,the ,indian lapidaries1 who were the first to realize that diamond could be ground with its own powder1 discovered what a wonderful difference the removal of the skin makes in the appearance of a stone4 ,they1 however1 made no attempt to shape a stone1 but merely polished the natural facets1 and only added numerous small facets when they wished to conceal flaws or other imperfections2 indeed1 the famous traveller1 ,tavernier1 from whom most of our knowledge of early mining in ,india is obtained1 invariably found that a stone covered with many facets was badly flawed4 ,the full radiant beauty of a diamond comes to light only when it is cut in brilliant form4 ,of all precious stones diamond has the simplest composition2 it is merely crystallized carbon1 another form of which is the humble and useful graphite1 commonly known as #8black-lead4' ,surely nature has surpassed all her marvellous efforts in producing from the same element substances with such divergent characters as the hard1 brilliant1 and transparent diamond and the soft1 dull1 and opaque graphite4 ,it is1 however1 impossible to draw any sharp dividing line between the two2 soft diamond passes insensibly into hard graphite1 and vice versa4 ,boart1 or bort1 as it is sometimes written1 is composed of minute crystals of diamond arranged haphazardly2 it possesses no cleavage1 its hardness is greater than that of the crystals1 and its colour is greyish to blackish4 ,carbon1 carbonado1 or black diamond1 which is composed of still more minute crystals1 is black and opaque1 and is perceptibly harder than the crystals4 ,it passes into graphite1 which varies in hardness1 and may have any density between #b,o and #c,o4 ,jewellers apply the term boart to crystals or fragments which are of no service as gems2 such pieces are crushed to powder and used for cutting and polishing purposes4 ,diamonds1 when absolutely limpid and free from flaws1 are said to be of the #8first water1' and are most prized when devoid of any tinge of colour except perhaps bluish 7,plate ,i1 ,fig4 #a74 ,stones with a slight tinge of yellow are termed #8off-coloured1' and are far less valuable4 ,those of a canary-yellow colour 7,plate ,i1 ,fig4 #c71 however1 belong to a different category1 and have a decided attractiveness4 ,greenish stones also are common1 though it is rare to come across one with a really good shade of that colour4 ,brown stones1 especially in ,south ,africa1 are not uncommon4 ,pink stones are less common1 and ruby-red and blue stones are rare4 ,those of the last-named colour have usually what is known as a #8steely' shade1 i4e4 they are tinged with green2 stones of a sapphire blue are very seldom met with1 and such command high prices4 ,illustration3 ,,figs4 #eg--ei4--;,diamond ,crystals4 ,diamond crystallizes 7,figs4 #eg--ei and ,plate ,i1 ,fig4 #b7 in octahedra with brilliant1 smooth faces1 and occasionally in cubes with rough pitted faces2 sometimes three or six faces take the place of each octahedron face1 and the stone is almost spherical in shape4 ,the surfaces of the crystals are often marked with equilateral triangles1 which are supposed to represent the effects of incipient combustion4 ,twinned crystals1 in which the two individuals may be connected by a single plane or may be interpenetrating1 a star shape often resulting in the latter case1 are common2 sometimes1 if of the octahedron type1 they are beautifully symmetrical4 ,the rounded crystals are frequently covered with a peculiar gum-like skin which is somewhat less hard than the crystal itself4 ,a large ,south ,african stone1 weighing #bg grams 7#acj carats7 and octahedral in shape1 which was the gift of ,john ,ruskin1 and named by him the #8,colenso' after the first bishop of ,natal1 is exhibited in the ,british ,museum 7,natural ,history72 its appearance is1 however1 marred by its distinctly #8off-coloured' tint4 ,the refraction of diamond is single1 but local double refraction is common1 indicating a state of strain which can often be traced to an included drop of liquid carbonic acid2 so great is the strain that many a fine stone has burst to fragments on being removed from the ground in which it has lain4 ,the refractive index for the yellow light of a sodium flame is #bdage1 and the slight variation from this mean value that has been observed1 amounting only to #jjjja1 testifies to the purity of the composition4 ,the colour-dispersion is large1 being as much as #jjdd1 in which respect it surpasses all colourless stones1 but is exceeded by sphene and the green garnet from the ,urals 7cf4 p4 #bag74 ,the lustre of diamond1 when polished1 is so characteristic as to be termed adamantine1 and is due to the combination of high refraction and extreme hardness4 ,diamond is translucent to the ,x 7,rntgen7 rays2 it phosphoresces under the action of radium1 and of a high-tension electric current when placed in a vacuum tube1 and sometimes even when exposed to strong sunlight4 ,some diamonds fluoresce in sunlight1 turning milky1 and a few even emit light when rubbed4 ,crookes found that a diamond buried in radium bromide for a year had acquired a lovely blue tint1 which was not affected even by heating to redness4 ,the specific gravity is likewise constant1 being #ceba1 with a possible variation from that mean value of #jjje2 but a greater range1 as might be expected1 is found in the impure boart4 ,diamond is by far the hardest substance in nature1 being marked #aj in ,mohs's scale of hardness1 but it varies in itself2 stones from ,borneo and ,new ,south ,wales are so perceptibly harder than those usually in the lapidaries' hands1 that they can be cut only with their own and not ordinary diamond powder1 and some difficulty was experienced in cutting them when they first came into the market4 ,it is interesting to note that the metal tantalum1 the isolation of which in commercial amount constituted one of the triumphs of chemistry of recent years1 has about the same hardness as diamond4 ,despite its extreme hardness diamond readily cleaves under a heavy blow in planes parallel to the faces of the regular octahedron1 a property utilized for shaping the stone previous to cutting it4 ,the fallacious1 but not unnatural1 idea was prevalent up to quite modern times that a diamond would1 even if placed on an anvil1 resist a blow from a hammer3 who knows how many fine stones have succumbed to this illusory test8 ,the fact that diamond could be split was known to ,indian lapidaries at the time of ,tavernier's visit1 and it would appear from ,de ,boodt that in the sixteenth century the cleavability of diamond was not unknown in ,europe1 but it was not credited at the time and was soon forgotten4 ,early last century ,wollaston1 a famous chemist and mineralogist1 rediscovered the property1 and1 so it is said1 used his knowledge to some profit by purchasing large stones1 which because of their awkward shape or the presence of flaws in the interior were rejected by the lapidaries1 and selling them back again after cleaving them to suitable forms4 ,it has already been remarked 7p4 #gi7 that the interval in hardness between diamond and corundum1 which comes next to it in ,mohs's scale1 is enormously greater than that between corundum and the softest of minerals4 ,diamond can therefore be cut only with the aid of its own powder1 and the cutting of diamond is therefore differentiated from that of other stones1 the precious-stone trade being to a large extent divided into two distinct groups1 namely1 dealers in diamonds1 and dealers in all other gem-stones4 ,the name of the species is derived from the popular form1 adiamentem1 of the ,latin adamantem1 itself the alliterative form of the ,greek 1 meaning the unconquerable1 in allusion not merely to the great hardness but also to the mistaken idea already mentioned4 ,boart probably comes from the ,old-,french bord or bort1 bastard4 ,at the present day diamonds are usually cut as brilliants1 though the contour of the girdle may be circular1 oval1 or drop-shaped to suit the particular purpose for which the stone is required1 or to keep the weight as great as possible4 ,small stones for bordering a large coloured stone may also be cut as roses or points4 ,a perfect brilliant has #eh facets1 but small stones may have not more than #dd1 and exceptionally large stones may with advantage have many more2 for instance1 on the largest stone cut from the ,cullinan diamond there are no fewer than #gd facets4 ,the description of the properties of diamond would not be complete without a reference to the other valuable1 if utilitarian1 purposes to which it is put4 ,without its aid much of modern engineering work and mining operations would be impossible except at the cost of almost prohibitive expenditure of time and money4 ,boring through solid rock has been greatly facilitated by the use of the diamond drill4 ,for this purpose carbonado or black diamond is more serviceable than single crystals1 and the price of the former has consequently advanced from a nominal figure up to #c to #ab a carat4 ,the actual working part of the drill consists of a cast-steel ring4 ,the crown of it has a number of small depressions at regular intervals into which the carbonados are embedded4 ,on revolution of the drill an annular ring is cut1 leaving a solid core which can be drawn to the surface4 ,for cooling the drill and for washing away the detritus water is pumped through to the working face4 ,the duration of the carbonados depends on the nature of the rock and the skill of the operator4 ,the most troublesome rock is a sandstone or one with sharp differences in hardness1 because the carbonados are liable to be torn out of their setting4 ,an experienced operator can tell by the feel of the drill the nature of the rock at the working face1 and by varying the pressure can mitigate the risk of damage to the drill4 ,the tenacity of diamond renders it most suitable for wire-drawing4 ,the tungsten filaments used in many of the latest forms of incandescent electric lamps are prepared in this manner4 ,diamond powder is used for cutting and turning the hardened steel employed in modern armaments and for other more peaceful purposes4 ,although nearly all the gem-stones scratch glass1 diamond alone can be satisfactorily employed to cut it along a definite edge4 ,any flake at random will not be suitable1 because it will tear the glass and form a jagged edge4 ,the best results are given by the junction of two edges which do not meet in too obtuse an angle2 two edges of the rhombic dodecahedron meet the requirements admirably4 ,the stones used by the glaziers are minute in size1 being not much larger than a pin's head1 and thirty of them on an average go to the carat4 ,they are set in copper or brass4 ,some little skill is needed to obtain the best results4 ,the value of a diamond has always been determined largely by the size of the stone1 the old rule being that the rate per carat should be multiplied by the square of the weight in carats2 thus1 if the rate be #aj1 the cost of a two-carat stone is four times this sum1 or #dj1 of a three-carat stone #ij1 and so on4 ,for a century1 from #agej to #ahej1 the rate remained almost constant at #d for rough1 #f for rose-cut1 and #h for brilliant-cut diamonds4 ,since the latter date1 owing to the increase in the supply of gold1 the growth of the spending power of the world1 and the gradual falling off in the productiveness of the ,brazilian fields1 the rate steadily increased about #aj per cent4 each year1 until in #ahfe the rate for brilliants was #ah4 ,the rise was checked by the discovery of the ,south ,african mines2 moreover4 since comparatively large stones are plentiful in these mines1 the rule of the increase in the price of a stone by the square of its weight no longer holds4 ,the rate for the most perfect stones still remains high1 because such are not so common in the ,south ,african mines4 ,the classification#g adopted by the syndicate of ,london diamond merchants who place upon the market the output of the ,de ,beers group of mines is as follows3--7a7 ,blue-white1 7b7 white1 7c7 silvery ,cape1 7d7 fine ,cape1 7e7 ,cape1 7f7 fine bywater1 7g7 bywater1 7h7 fine light brown1 7i7 light brown1 7j7 brown1 7k7 dark brown4 ,bywaters or byes are stones tinged with yellow4 ,the rate per carat for cut stones in the blue-white and the bywater groups is3-- ,,blue-,,white4 ,,bywater4 #e-;carat stone #dj-fj #bj-be #a #cj-dj #aj-ae #bj-be #h-ab #ae-ah #f-aj ,mle #ab-ae #e-h ,mle are stones smaller than a quarter of a carat4 ,it will be noticed that the prices depart largely from the old rule2 thus taking the rate for a carat blue-white stone1 the price of a five-carat stone should be from #aej-bjj a carat1 and for a quarter-carat stone only #g1 #ajs4 to #aj a carat4 ,there happens to be at the time of writing very little demand for five-carat stones4 ,of course1 the prices given are subject to constant fluctuation depending upon the supply and demand1 and the whims of fashion4 ,,chapter ,,xvii ,,occurrence ,,of ,,diamond ,the whole of the diamonds known in ancient times were obtained from the so-called ,golconda mines in ,india4 ,golconda itself1 now a deserted fortress near ,hyderabad1 was merely the mart where the diamonds were bought and sold4 ,the diamond-bearing district actually spread over a wide area on the eastern side of the ,deccan1 extending from the ,pinner ,river in the ,madras ,presidency northwards to the ,rivers ,son and ,khan1 tributaries of the ,ganges1 in ,bundelkhand4 ,the richest mines1 where the large historical stones were found1 are in the south1 mostly near the ,kistna ,river4 ,the diamonds were discovered in sandstone1 or conglomerate1 or the sands and gravels of river-beds4 ,the mines were visited in the middle of the seventeenth century by the ,french traveller and jeweller1 ,tavernier1 when travelling on a commission for ,louis ,,xiv1 and he afterwards published a careful description of them and of the method of working them4 ,the mines seem to have been exhausted in the seventeenth century2 at any rate1 the prospecting1 which has been spasmodically carried on during the last two centuries1 has proved almost abortive4 ,with the exception of the ,koh-i-nor1 all the large ,indian diamonds were probably discovered not long before ,tavernier's visit4 ,the diamonds known to ,pliny1 and in his time1 were quite small1 and it is doubtful if any stones of considerable size came to light before ,a4,d4 #ajjj4 ,india enjoyed the monopoly of supplying the world's demand for diamonds up to the discovery1 in #agbe1 of the precious stone in ,brazil4 ,small stones were detected by the miners in the gold washings at ,tejuco1 about eighty miles 7#abi km47 from ,rio de ,janeiro1 in the ,serro do ,frio district of the ,state of ,minas ,geraes4 ,the discovery naturally caused great excitement4 ,so many diamonds were found that in #agbg something like a slump took place in their value4 ,in order to keep up prices1 the ,dutch merchants1 who mainly controlled the ,indian output1 asserted that the diamonds had not been found in ,brazil at all1 but were inferior ,indian stones shipped to ,brazil from ,goa4 ,the tables were neatly turned when diamonds were actually shipped from ,brazil to ,goa1 and exported thence to ,europe as ,indian stones4 ,this course and the continuous development of the diamond district in ,brazil rendered it impossible to hoodwink the world indefinitely4 ,the drop in prices was1 however1 stayed by the action of the ,portuguese government1 who exacted such heavy duties and imposed such onerous conditions that finally no one would undertake to work the mines4 ,accordingly1 in #aggb diamond-mining was declared a royal monopoly in ,brazil1 and such it remained until the severance of ,brazil from ,portugal in #ahcd1 when private mining was permitted by the new government subject to the payment of reasonable royalties4 ,the industry was enormously stimulated by the discovery1 in #ahdd1 of the remarkably rich fields in the ,state of ,bahia1 especially at ,serra da ,cincor1 where carbonado1 or black diamond1 first came to light1 but after a few years1 owing to the difficulties of supplying labour1 the unhealthiness of the climate1 and the high cost of living1 the yield fell off and gradually declined1 until the importance of the fields was finally eclipsed by the rise of the ,south ,african mines4 ,the ,brazilian mines have proved very productive1 but chiefly in small diamonds1 stones above a carat in weight being few in comparison4 ,the largest stone1 to which the name1 the ,star of the ,south1 was applied1 weighed in the rough #bed carats2 it was discovered at the ,bagagem mines in #ahec4 ,the quality of the diamonds is good1 many of them having the highly-prized bluish-white colour4 ,the principal diamond-bearing districts of ,brazil centre at ,diamantina1 as ,tejuco was re-named after the discovery of diamonds1 ,gro ,magor1 and ,bagagem in the ,state of ,minas ,geraes1 at ,diamantina in the ,state of ,bahia1 and at ,goyz and ,matto ,grosso in the ,states of the same names4 ,the diamonds occur chiefly in cascalho1 a gravel1 containing large masses of quartz and small particles of gold1 which is supposed to be derived from a quartzose variety of micaceous slate known as itacolumite4 ,the mines are now to some extent being worked by systematic dredging of the river-beds4 ,early in #ahfg the children of a ,boer farmer1 ,daniel ,jacobs1 who dwelt near ,hopetown on the banks of the ,orange ,river1 picked up in the course of play near the river a white pebble1 which was destined not only to mark the commencement of a new epoch in the record of diamond mines1 but to change the whole course of the history of ,south ,africa4 ,this pebble attracted the attention of a neighbour1 ,schalk van ,niekerk1 who suspected that it might be of some value1 and offered to buy it4 ,mrs4 ,jacobs1 however1 gave it him1 laughingly scouting the idea of accepting money for a mere pebble4 ,van ,niekerk showed it to a travelling trader1 by name ,john ,o',reilly1 who undertook to obtain what he could for it on condition that they shared the proceeds4 ,every one he met laughed to scorn the idea that the stone had any value1 and it was once thrown away and only recovered after some search in a yard1 but at length he showed it to ,lorenzo ,boyes1 the ,acting ,civil ,commissioner at ,colesberg1 who1 from its extreme hardness1 thought it might be diamond and sent it to the mineralogist1 ,w4 ,guybon ,atherston1 of ,grahamstown1 for determination4 ,so uncertain was ,boyes of its value that he did not even seal up the envelope containing it1 much less register the package4 ,atherston found immediately that the long-scorned pebble was really a fine diamond1 weighing #ba-c_/af carats1 and with ,o',reilly's consent he submitted it to ,sir ,philip ,wodehouse1 ,governor at the ,cape4 ,the latter purchased it at once for #ejj1 and dispatched it to be shown at the ,paris ,exhibition of that year4 ,it did not1 however1 attract much attention2 chimerical tales of diamond finds in remote parts of the world are not unknown4 ,indeed1 for some time only a few small stones were picked up beside the ,orange ,river1 and no one believed in the existence of any extensive diamond deposit4 ,however1 all doubt as to the advisibility of prospecting the district was settled by the discovery of the superb diamond1 afterwards known as the #8,star of ,south ,africa1' which was picked up in ,march #ahfi by a shepherd boy on the ,zendfontein farm near the ,orange ,river4 ,van ,niekerk1 on the alert for news of further discoveries1 at once hurried to the spot and purchased the stone from the boy for five hundred sheep1 ten oxen1 and a horse1 which seemed to the boy untold wealth1 but was not a tithe of the #aa1bjj which ,lilienfeld ,bros41 of ,hopetown1 gave ,van ,niekerk4 ,illustration3 ,,plate ,,xv ,,kimberley ,,mine1 #ahga ,illustration3 ,,plate ,,xvi ,,kimberley ,,mine1 #ahgb ,this remarkable discovery attracted immediate attention to the potentialities of a country which produced diamonds of such a size1 and prospectors began to swarm into the district1 gradually spreading up the ,vaal ,river4 ,for some little time not much success was experienced1 but at length1 early in #ahgj1 a rich find was made at ,klipdrift1 now known as ,barkly ,west1 which was on the banks of the ,vaal ,river immediately opposite the ,mission camp at ,pniel4 ,the number of miners steadily increased until the population on the two sides of the river included altogether some four or five thousand people1 and there was every appearance of stability in the existing order of things4 ,but a vast change came over the scene upon the discovery of still richer mines lying to the south-east and some distance from the river4 ,the ground was actually situated on the route traversed by parties hurrying to the ,vaal ,river1 but no one dreamed of the wealth that lay under their feet4 ,the first discovery was made in ,august #ahgj at the farm ,jagersfontein1 near ,fauresmith in ,orange ,river ,colony1 by ,de ,klerk1 the intelligent overseer1 who noticed in the dry bed of a stream a number of garnets1 and1 knowing that they often accompanied diamond1 had the curiosity to investigate the point4 ,he was immediately rewarded by finding a fine diamond weighing #ej carats4 ,in the following month diamonds were discovered about twenty miles from ,klipdrift at ,dutoitspan on the ,dorstfontein farm1 and a little later also on the contiguous farm of ,bultfontein2 a diamond was actually found in the mortar used in the homestead of the latter farm4 ,early in ,may #ahga diamonds were found about two miles away on ,de ,beers' farm1 ,vooruitzigt1 and two months later1 in ,july1 a far richer find was made on the same farm at a spot which was first named ,colesberg ,kopje1 the initial band of prospectors having come from the town of that name near the ,orange ,river1 but was subsequently known as ,kimberley after the ,secretary of ,state for the ,colonies at that time4 ,soon a large and prosperous town sprang up close to the mines2 it rapidly grew in size and importance1 and to this day remains the centre of the diamond-mining industry4 ,subsequent prospecting proved almost blank until the discovery of the ,premier or ,wesselton mine on ,wesselton farm1 about four miles from ,kimberley1 in ,september #ahij2 it received the former name after ,rhodes1 who was ,premier of ,cape ,colony at that date4 ,no further discovery of any importance was made until1 in #aijb1 diamonds were found about twenty miles north-west-north of ,pretoria in the ,transvaal1 at the new ,premier mine1 now famed as the producer of the gigantic ,cullinan diamond4 ,illustration3 ,,plate ,,xvii ,,kimberley ,,mine1 #ahgd ,illustration3 ,,plate ,,xviii ,,kimberley ,,mine1 #ahha ,the ,kimberley mines were at first known as the #8dry diggings' on account of their arid surroundings in contradistinction to the #8river diggings' by the ,vaal4 ,the dearth of water was at first one of the great difficulties in the way of working the former mines1 although subsequently the accumulation of underground water at lower levels proved a great obstacle to the working of the mines4 ,the #8river diggings' were of a type similar to that met with in ,india and ,brazil1 the diamonds occurring in a gravelly deposit of limited thickness beneath which was barren rock1 but the ,kimberley mines presented a phenomenon hitherto without precedent in the whole history of diamond mining4 ,the diamonds were found in a loose surface deposit1 which was easily worked1 and for some time the prospectors thought that the underlying limestone corresponded to the bedrock of the river gravel1 until at length one more curious than his fellows investigated the yellowish ground underneath1 and found to his surprise that it was even richer than the surface layer4 ,immediately a rush was made back to the deserted claims1 and the mines were busier than ever4 ,this #8yellow ground1' as it is popularly called1 was much decomposed and easy1 therefore1 to work and sift4 ,about fifty to sixty feet 7#ae-ah m47 below the surface1 however1 it passed into a far harder rock1 which from its colour is known as the #8blue ground'2 this also1 to the unexpected pleasure of the miners1 turned out to contain diamonds4 ,difficulties arose as each claim1 #cj by #cj ,dutch feet 7about #ca ,english feet or #ide metres square7 in area1 was worked downwards4 ,in the ,kimberley mine 7,plate ,,xvi7 access to the various claims was secured by retaining parallel strips1 #ae feet wide1 each claim being1 therefore1 reduced in width to #bb feet1 to form roadways running from side to side of the mine in one direction4 ,these1 however1 soon gave way1 not only because of the falling of the earth composing them1 but because they were undermined and undercut by the owners of the adjacent claims4 ,by the end of #ahgb the last roadway had disappeared1 and the mine presented the appearance of a vast pit4 ,in order to obtain access to the claims without intruding on those lying between1 and to provide for the hauling of the loads of earth to the surface1 an ingenious system of wire cables in three tiers 7,plate ,,xvii7 was erected1 the lowest tier being connected to the outermost claims1 the second to claims farther from the edge1 and the highest to claims in the centre of the pit4 ,the mine at that date presented a most remarkable spectacle1 resembling an enormous radiating cobweb1 which had a weird charm by night as the moonlight softly illuminated it1 and by day1 owing to the perpetual ring of the flanged wheels of the trucks on the running wires1 twanged like some gigantic olian harp4 ,this system fulfilled its purpose admirably until1 with increasing depth of the workings1 other serious difficulties arose4 ,deprived of the support of the hard blue ground1 the walls of the mine tended to collapse1 and additional trouble was caused by the underground water that percolated into the mine4 ,by the end of #ahhc the floor of the ,kimberley mine was almost entirely covered by falls of #8reef' 7,plate ,,xviii71 as the surrounding rocks are termed1 the depth then being about #djj feet 7#abb m474 ,in the ,de ,beers mine1 in spite of the precaution taken to prevent falls of reef by cutting the walls of the mine back in terraces1 falls occurred continuously in #ahhd1 and by #ahhg1 at a depth of #cej feet 7#ajg m471 all attempts at open working had to be abandoned4 ,in the ,dutoitspan mine buttresses of blue ground were left1 which held back the reef for some years1 but ultimately the mine became unsafe1 and in ,march #ahhf a disastrous fall took place1 in which eighteen miners--eight white men and ten ,kafirs--lost their lives4 ,the ,bultfontein mine was worked to the great depth of #ejj feet 7#aeb m471 but falls occurred in #ahhi and put an end to open working4 ,in all cases1 therefore1 the ultimate end was the same3 the floor of the mine became covered with a mass of worthless reef1 which rendered mining from above ground dangerous1 and1 indeed1 impossible except at prohibitive cost4 ,it was then clearly necessary to effect access to the diamond-bearing ground by means of shafts sunk at a sufficient distance from the mine to remove any fear of falls of reef4 ,for such schemes co-operative working was absolutely essential4 ,plate ,,xix illustrates the desolate character of the ,kimberley mine above ground and the vastness of the yawning pit1 which is over #ajjj feet 7#cjj m47 in depth4 ,illustration3 ,,plate ,,xix ,,kimberley ,,mine ,,at ,,the ,,present ,,day ,illustration3 ,,plate ,,xx ,,wesselton 7open7 ,,mine ,a certain amount of linking up of claims had already taken place1 but1 although many men must have seen that the complete amalgamation of the interests in each mine was imperative1 two men alone had the capacity to bring their ideas to fruition4 ,c4 ,j4 ,rhodes was the principal agent in the formation in ,april #ahhj of the ,de ,beers ,mining ,company1 which rapidly absorbed the remaining claims in the mine1 and was re-formed in #ahhg as the ,de ,beers ,consolidated ,mining ,company4 ,meantime1 ,barnett ,isaacs1 better known by the cognomen ,barnato1 which had been adopted by his brother ,henry when engaged in earning his livelihood in the diamond fields as an entertainer1 had secured the major interests in the ,kimberley mine4 ,rhodes saw that1 for effective working of the two mines by any system of underground working1 they must be under one management1 but to all suggestions of amalgamation ,barnato remained deaf1 and at last ,rhodes determined to secure control of the ,kimberley mine at all costs4 ,the story of the titanic struggle between these two men forms one of the epics of finance4 ,eventually1 when shares in the ,kimberley mine had been boomed to an extraordinary height1 and the price of diamonds had fallen as low as #ahs4 a carat1 ,barnato gave way1 and in ,july #ahhi the ,kimberley mine was absorbed by the ,de ,beers ,company on payment of the enormous sum of #e1cch1fej4 ,shortly afterwards they undertook the working of the ,dutoitspan and the ,bultfontein mines1 and in ,january #ahif they acquired the ,premier or ,wesselton mine4 ,the interests in the ,jagersfontein mine were in #ahhh united in the ,new ,jagersfontein ,mining and ,exploration ,company1 and the mine is now worked also by the ,de ,beers ,company4 ,thus1 until the development of the new ,premier mine in the ,transvaal1 the ,de ,beers ,company practically controlled the diamond market4 ,the development of this last mine was begun so recently1 and its size is so vast--the longest diameter being half a mile--that open-cut working is likely to continue for some years4 ,illustration3 ,,plate ,,xxi ,,loading ,,the ,,blue ,,ground ,,on ,,the ,,floors1 ,,and ,,ploughing ,,it ,,over ,illustration3 ,,plate ,,xxii ,,washing-,,machines ,,for ,,concentrating ,,the ,,blue ,,ground ,though varying slightly in details1 the methods of working the mines are identical in principle4 ,from the steeply inclined shaft horizontal galleries are run diagonally right across the mine1 the vertical interval between successive galleries being #dj feet4 ,from each gallery side galleries are run at right angles to it and parallel to the working face4 ,the blue ground is worked systematically backwards from the working face4 ,the mass is stoped1 i4e4 drilled and broken from the bottom upwards1 until only a thin roof is left4 ,as soon as the section is worked out and the material removed1 the roof is allowed to fall in1 and work is begun on the next section of the same level2 at the same time the first section on the level next below is opened out4 ,thus work is simultaneously carried on in several levels1 and a vertical plane would intersect the working faces in a straight line obliquely inclined to the vertical direction 7,fig4 #fj74 ,when freshly mined1 the blue ground is hard and compact1 but it soon disintegrates under atmospheric influence4 ,indeed1 the yellow ground itself was merely decomposed blue ground4 ,no immediate attempt is made1 therefore1 to retrieve the precious stones4 ,the blue ground is spread on to the #8floors' 7,plate ,,xxi71 i4e4 spaces of open veldt which have been cleared of bushes and inequalities1 to the depth of a couple of feet1 and remains there for periods ranging from six months to two years1 depending on the quality of the blue ground and the amount of rainfall4 ,to hasten the disintegration the blue ground is frequently ploughed over and occasionally watered1 a remarkable introduction of agricultural methods into mining operations4 ,no elaborate patrolling or guarding is required1 because the diamonds are so sparsely1 though regularly1 scattered through the mass that even of the actual workers in the mines but few have ever seen a stone in the blue ground4 ,when sufficiently broken up1 it is carted to the washing and concentrating machines1 by means of which the diamonds and the heavier constituents are separated from the lighter material4 ,illustration3 ,,fig4 #fj4--,vertical ,section of ,diamond ,pipe1 showing ,tunnels and ,stopes4 ,formerly the diamonds were picked out from the concentrates by means of the keen eyes of skilled natives2 but the process has been vastly simplified and the risk of theft entirely eliminated by the remarkable discovery made in #ahig by ,f4 ,kirsten1 of the ,de ,beers ,company1 that of all the heavy constituents of the blue ground diamond alone1 with the exception of an occasional corundum and zircon1 which are easily sorted out afterwards1 adheres to grease more readily than to water4 ,in this ingenious machine1 the #8jigger' or #8greaser' 7,plate ,,xxiii7 as it is commonly termed1 the concentrates are washed over a series of galvanized-iron trays1 which are covered with a thick coat of grease4 ,the trays are slightly inclined downwards1 and are kept by machinery in constant sideways motion backwards and forwards4 ,so accurate is the working of this device that few diamonds succeed in getting beyond the first tray1 and none progress as far as the third1 which is added as an additional precaution4 ,the whole apparatus is securely covered in so that there is no risk of theft during the operation4 ,the trays are periodically removed1 and the grease is scraped off and boiled to release the diamonds1 the grease itself being used over again on the trays4 ,this is the first time in the whole course of extraction from the mines that the diamonds are actually handled4 ,the stones are now passed on to the sorters1 who separate them into parcels according to their size1 shape1 and quality4 ,illustration3 ,,plate ,,xxiii ,,diamond-,,sorting ,,machines ,illustration3 ,,plate ,,xxiv ,,kafirs ,,picking ,,out ,,diamonds ,the classification at the mines is first into groups by the shape3 7#a7 close goods1 7#b7 spotted stones1 7#c7 rejection cleavage1 7#d7 fine cleavage1 7#e7 light brown cleavage1 7#f7 ordinary and rejection cleavage1 7#g7 flats1 7#h7 macles1 7#i7 rubbish1 7#aj7 boart4 ,close goods are whole crystals which contain no flaws and can be cut into single stones4 ,spotted stones1 as their name suggests1 contain spots which necessitate removal1 and cleavage includes stones which are so full of flaws that they have to be cleaved or split into two or more stones4 ,flats are distorted octahedra1 and macles are twinned octahedra4 ,rubbish is material which can be utilized only for grinding purposes1 and boart consists of round dark stones which are invaluable for rock-drills4 ,these groups are afterwards graded into the following subdivisions1 depending on increasing depth of yellowish tint3 7a7 blue-white1 7b7 first ,cape1 7c7 second ,cape1 7d7 first bye1 7e7 second bye1 7f7 off-colour1 7g7 light yellow1 7h7 yellow4 ,it is1 however1 only the first group that is so minutely subdivided4 ,after being purchased1 the parcels are split up again somewhat differently for the ,london market 7cf4 p4 #acf71 and the dealers re-arrange the stones according to the purpose for which they are required4 ,formerly a syndicate of ,london merchants took the whole of the produce of the ,kimberley mines at a previously arranged price per carat1 but at the present time the diamonds are sold by certain ,london firms on commission4 ,the products of each mine show differences in either form or colour which enable an expert readily to recognize their origin4 ,the old diggings by the ,vaal ,river yielded finer and more colourless stones than those found in the dry diggings and the mines underlying them4 ,the ,south ,african diamonds1 taken as a whole1 are always slightly yellowish or #8off-coloured'2 the mines are1 indeed1 remarkable for the number of fine and large1 canary-yellow and brown1 stones produced4 ,the ,kimberley mine yields a fair percentage of white1 and a large number of twinned and yellow stones4 ,the yield of the ,de ,beers mine comprises mostly tinted stones--yellow and brown1 occasionally silver capes1 and very seldom stones free from colour4 ,the ,dutoitspan mine is noted for its harvest of large yellow diamonds2 it also produces fine white cleavage and small white octahedra4 ,the stones found in the ,bultfontein mine are small and spotted1 but1 on the other hand1 the yield has been unusually regular4 ,the ,premier or ,wesselton mine yields a large proportion of flawless octahedra1 but1 above all1 a large number of beautiful deep-orange diamonds4 ,of all the ,south ,african mines the ,jagersfontein in the ,orange ,river ,colony alone supplies stones of the highly-prized blue-white colour and steely lustre characteristic of the old ,indian stones4 ,the new ,premier mine in the ,transvaal is prolific1 but mostly in off-coloured and low-grade stones1 the ,cullinan diamond being a remarkable exception4 ,to illustrate the amazing productiveness of the ,south ,african mines1 it may be mentioned that1 according to ,gardner ,f4 ,williams1 the ,kimberley group of mines in sixteen years yielded #cf million carats of diamonds1 and the annual output of the ,jagersfontein mine averages about a quarter of a million carats1 whereas the total output of the ,brazil mines1 for the whole of the long period during which they have been worked1 barely exceeds #ac million carats4 ,the average yield of the ,south ,african mines1 however1 perceptibly diminishes as the depth of the mines increases4 ,the most interesting point connected with the ,south ,african diamond mines1 viewed from the scientific standpoint1 is the light that they have thrown on the question of the origin of the diamond1 which previously was an incomprehensible and apparently insoluble problem4 ,in the older mines1 just as at the river diggings by the ,vaal1 the stones are found in a gravelly deposit that has resulted from the disintegration of the rocks through which the adjacent river has passed1 and it is clear that the diamond cannot have been formed in situ here2 it had been suspected1 and now there is no doubt1 that the itacolumite rock of ,brazil has consolidated round the diamonds which are scattered through it1 and that it cannot be the parent rock4 ,the occurrence at ,kimberley is very different4 ,these mines are funnels which go downwards to unknown depths2 they are more or less oval in section1 becoming narrower with increasing depth1 and are evidently the result of some eruptive agency4 ,the ,kimberley mine has been worked to a depth of nearly #djjj feet 7#abjj m471 and no signs of a termination have as yet appeared4 ,the blue ground which fills these #8pipes1' as they are termed1 must have been forced up from below1 since it is sharply differentiated from the surrounding country rocks4 ,this blue ground is a brecciated peridotite of peculiar constitution1 to which the well-known petrologist1 ,carvil ,lewis1 who made a careful study of it1 gave the name kimberlite4 ,the blue colour testifies to its richness in iron1 and it is to the oxidation of the iron constituent1 that the change of colour to yellow in the upper levels is due4 ,owing to the shafts that have been sunk for working the mines1 the nature of the surrounding rocks is known to some depth4 ,immediately below the surface is a decomposed ferriferous basalt1 about #bj to #ij feet 7#f-bg m47 thick1 next a black slaty shale1 #bjj to #bej feet 7#fj-ge m47 thick1 then #aj feet 7#c m47 of conglomerate1 next #djj feet 7#abj m47 of olivine diabase1 then quartzite1 about #djj feet 7#abj m47 thick1 and lastly a quartz porphyry1 which has not yet been penetrated4 ,the strata run nearly horizontal1 and there are no signs of upward bending at the pipes4 ,the whole of the country1 including the mines1 was covered with a red sandy soil1 and there was nothing to indicate the wealth that lay underneath4 ,the action of water had in process of time removed all signs of eruptive activity4 ,the principal minerals which are associated with diamond in the blue ground are magnetite1 ilmenite1 chromic pyrope1 which is put on the market as a gem under the misnomer #8,cape-ruby1' ferriferous enstatite1 which also is sometimes cut1 olivine more or less decomposed1 zircon1 kyanite1 and mica4 ,the evidence produced by an examination of the blue ground and the walls of the pipes proves that the pipes cannot have been volcanoes such as ,vesuvius4 ,there is no indication whatever of the action of any excessive temperature1 while1 on the other hand1 there is every sign of the operation of enormous pressure2 the diamonds often contain liquid drops of carbonic acid4 ,crookes puts forward the plausible theory that steam has been the primary agency in propelling the diamond and its associates up into the channel through which it has carved its way to freedom1 and holds that molten iron has been the solvent for carbon which has crystallized out as diamond under the enormous pressures obtaining in remote depths of the earth's crust4 ,it is pertinent to note that1 by dissolving carbon in molten iron1 the eminent chemist1 ,moissan1 was enabled to manufacture tiny diamond crystals4 ,water trickling down from above would be immediately converted into steam at very high pressure on coming into contact with the molten iron1 and1 in its efforts to escape1 the steam would drive the iron and its precious contents1 together with the adjacent rocks1 upwards to the surface4 ,the ferriferous nature of the blue ground and the yellow tinge so common to the diamonds lend confirmation to this theory4 ,the process by which the carbon was extracted from shales or other carboniferous rocks and dissolved in iron still awaits elucidation4 ,diamonds were found in ,new ,south ,wales as long ago as #ahea on ,turon ,river and at ,reedy ,creek1 near ,bathurst1 about ninety miles 7#ade km47 from ,sydney1 but the find was of little commercial importance4 ,a more extensive deposit came to light in #ahfg farther north at ,mudgee4 ,in #ahgb diamonds were discovered in the extreme north of the ,state1 at ,bingara near the ,queensland border4 ,another discovery was made in #ahhd at ,tingha1 and still more recently in the tin gravels of ,inverell in the same region4 ,in their freedom from colour and absence of twinning the ,new ,south ,wales diamonds resemble the ,brazilian stones4 ,the average size is small1 running about five to the carat when cut2 the largest found weighed nearly #f carats when cut4 ,they are remarkable for their excessive hardness2 they can be cut only with their own dust1 ordinary diamond dust making no impression4 ,the ,borneo diamonds are likewise distinguished by their exceptional hardness4 ,they mostly occur by the river ,landak1 near ,pontianak on the west coast of the island4 ,they are found in a layer of rather coarse gravel1 variable1 but rarely exceeding a yard 7#a m471 in depth1 and are associated with corundum and rutile1 together with the precious metals gold and platinum4 ,indeed1 it is no uncommon sight to see natives wearing waistcoats ornamented with gold buttons1 in each of which a diamond is set4 ,the diamonds are well crystallized and generally of pure water2 yellowish and canary-yellow stones are also common1 but rose-red1 bluish1 smoky1 and black stones are rare4 ,they seldom exceed a carat in weight2 but stones of #aj carats in weight are found1 and occasionally they attain to #bj carats4 ,in #ahej a diamond weighing #gg carats was discovered4 ,the ,rajah of ,mattan is said to possess one of the purest water weighing as much as #cfg carats1 but no one qualified to pronounce an opinion regarding its genuineness has ever seen it4 ,in ,rhodesia small diamonds have been found in gravel beds resting on decomposed granite near the ,somabula forest1 about #ab miles 7#ai km47 west of ,gwelo1 in association with chrysoberyl in abundance1 blue topaz1 kyanite1 ruby1 sapphire1 tourmaline1 and garnet4 ,the occurrence of diamond in ,german ,south-,west ,africa is very peculiar4 ,large numbers of small stones are found close to the shore near ,luderitz ,bay in a gravelly surface layer1 which is nowhere more than a foot in depth4 ,they are picked by hand by natives and washed in sieves4 ,in shape they are generally six-faced octahedra or twinned octahedra1 simple octahedra being rare1 and in size they run about four or five to the carat1 the largest stone as yet found being only #b carats in weight4 ,their colour is usually yellowish4 ,several isolated finds of diamonds have been reported in ,california and other parts of the ,united ,states1 but none have proved of any importance4 ,the largest stone found weighed #bc carats uncut2 it was discovered at ,manchester in ,virginia4 ,,chapter ,,xviii ,,historical ,,diamonds ,the number of diamonds which exceed a hundred carats in weight when cut is very limited4 ,their extreme costliness renders them something more than mere ornaments2 in a condensed and portable form they represent great wealth and all the potentiality for good or ill thereby entailed1 and have played no small1 if sinister1 rle in the moulding of history4 ,in bygone days when despotic government was universal1 the possession of a splendid jewel in weak hands but too often precipitated the aggression of a greedy and powerful neighbour1 and plunged whole countries into the horrors of a ruthless and bloody war4 ,in more civilized days a great diamond has often been pledged as security for money to replenish an empty treasury in times of stress4 ,the ambitions of ,napoleon might have received a set-back but for the funds raised on the security of the famous ,pitt diamond4 ,the history of such stones--often one long romance--is full of interest1 but space will not permit of more than a brief sketch here4 ,if we except the colossal ,cullinan stone1 the mines of ,brazil and ,south ,africa cannot compare with the old mines of ,india as the birthplace of large and perfect diamonds of world-wide fame4 7#a7 ,,koh-,i-,,nor ,illustration3 ,,fig4 #fa4--,koh-i-nor 7top view74 ,illustration3 ,,fig4 #fb4--,koh-i-nor 7side view74 ,the history of the famous stone called the ,koh-i-nor1 meaning ,mound of ,light1 is known as far back as the year #acjd1 when it fell into the hands of the ,mogul emperors1 and legend even traces it back some four thousand years previously4 ,it remained at ,delhi until the invasion of ,north-,west ,india by ,nadir ,shah in #agci1 when it passed together with an immense amount of spoil into the hands of the conqueror4 ,at his death the empire which he had so strenuously founded fell to pieces1 and the great diamond after many vicissitudes came into the possession of ,runjit ,singh at ,lahore4 ,his successors kept it until upon the fall of the ,sikh power in #ahej it passed to the ,east ,india ,company1 in whose name it was presented by ,lord ,dalhousie to ,queen ,victoria4 ,at this date the stone still retained its original ,indian form1 but in #ahfb it was re-cut into the form of a shallow brilliant 7,fig4 #fb71 the weight thereby being reduced from #ahf-a_/af to #ajf-a_/af carats4 ,the wisdom of this course has been severely criticized2 the stone has not the correct shape of a brilliant and is deficient in #8fire1' and it has with the change in shape lost much of its old historical interest4 ,the ,koh-i-nor is the private property of the ,english ,royal ,family1 the stone shown in the ,tower being a model4 ,it is valued at #ajj1jjj4 7#b7 ,,pitt ,,or ,,regent ,illustration3 ,,fig4 #fc4--,pitt or ,regent 7top view74 ,illustration3 ,,fig4 #fd4--,pitt or ,regent 7side view74 ,this splendid stone was discovered in #agja at the famous diamond mines at ,partial1 on the ,kistna1 about #aej miles 7#bdj km47 from ,golconda1 and weighed as much as #daj carats in the rough4 ,by devious ways it came into the hands of ,jamchund1 a ,parsee merchant1 from whom it was purchased by ,william ,pitt1 governor of ,fort ,st4 ,george1 ,madras1 for #bj1djj4 ,on his return to ,england ,pitt had it cut into a perfect brilliant 7,fig4 #fc71 weighing #afc carats1 the operation occupying the space of two years and costing #ejjj2 more than #gjjj is said to have been realized from the sale of the fragments left over4 ,pitt had an uneasy time and lived in constant dread of theft of the stone until1 in #agag1 after lengthy negotiations1 he parted with it to the ,duc d',orlans1 ,regent of ,france1 for the immense sum of three and three-quarter million francs1 about #ace1jjj4 ,with the remainder of the ,french regalia it was stolen from the ,garde-meuble on ,august #ag1 #agib1 in the early days of the ,french ,revolution1 but was eventually restored by the thieves1 doubtless because of the impossibility of disposing of such a stone1 at least intact1 and it is now exhibited in the ,apollo ,gallery of the ,louvre at ,paris4 ,it measures about #cj millimetres in length1 #be in width1 and #ai in depth1 and is valued at #dhj1jjj4 7#c7 ,,orloff ,illustration3 ,,fig4 #fe4--,orloff 7top view74 ,illustration3 ,,fig4 #ff4--,orloff 7side view74 ,one of the finest diamonds existing1 this large stone forms the top of the imperial sceptre of ,russia4 ,it is rose-cut 7,fig4 #fe71 the base being a cleavage face1 and weighs #aid carats4 ,it is said to have formed at one time one of the eyes of a statue of ,brahma which stood in a temple on the island of ,sheringham in the ,cavery ,river1 near ,trichinopoli1 in ,mysore1 and to have been stolen by a ,french soldier who had somehow persuaded the priests to appoint him guardian of the temple4 ,he sold it for #bjjj to the captain of an ,english ship1 who disposed of it to a ,jewish dealer in ,london for #ab1jjj4 ,it changed hands to a ,persian merchant1 ,raphael ,khojeh1 who eventually sold it to ,prince ,orloff for1 so it is said1 the immense sum of #ij1jjj and an annuity of #djjj4 ,it was presented by ,prince ,orloff to ,catherine ,,ii of ,russia4 7#d7 ,,great ,,mogul ,this1 the largest ,indian diamond known1 was found in the ,kollur mines1 about the year #afej4 ,its original weight is said to have been #ghg carats1 but it was so full of flaws that the ,venetian1 ,hortensio ,borgis1 then in ,india1 in cutting it to a rose form reduced its weight to #bdj carats4 ,it was seen by ,tavernier at the time of his visit to ,india1 but it has since been quite lost sight of4 ,it has been identified with both the ,koh-i-nor and the ,orloff1 and it is even suggested that both these stones were cut from it4 7#e7 ,,sancy ,the history of this diamond is very involved1 and probably two or more stones have been confused4 ,it may have been the one cut by ,berquem for ,charles the ,bold1 from whose body on the fatal day of ,nancy1 in #adgg1 it was snatched by a marauding soldier4 ,it was acquired by ,nicholas ,harlai1 ,seigneur de ,sancy1 who sold it to ,queen ,elizabeth at the close of the sixteenth century4 ,a hundred years later1 in #afie1 it was sold by ,james ,,ii to ,louis ,,xiv4 ,the stone in the ,french regalia1 according to the inventory taken in #agia1 weighed #ec carats4 ,it was never recovered after the theft of the regalia in the following year1 but may be identical with the diamond which was in the possession of the ,demidoff family and was sold by ,prince ,demidoff in #ahfe to a ,london firm who were said to have been acting for ,sir ,jamsetjee ,jeejeebhoy1 a wealthy ,parsee of ,bombay4 ,it was shown at the ,paris ,exhibition of #ahfg4 ,it was almond-shaped1 and covered all over with tiny facets by ,indian lapidaries4 7#f7 ,,great ,,table ,this mysterious stone was seen by ,tavernier at ,golconda in #afdb1 but has quite disappeared4 ,it weighed #bdb-c_/af carats4 7#g7 ,,moon ,,of ,,the ,,mountains ,this diamond is often confused with the ,orloff4 ,it was captured by ,nadir ,shah at ,delhi1 and after his murder was stolen by an ,afghan soldier who disposed of it to an ,armenian1 by name ,shaffrass4 ,it was finally acquired by the ,russian crown for an enormous sum4 7#h7 ,,nizam ,a large diamond1 weighing #cdj carats1 belonged to the ,nizam of ,hyderabad2 it was fractured at the beginning of the ,indian ,mutiny4 ,whether the weight is that previous to fracture or not1 there seems to be no information4 7#i7 ,,darya-,i-,,nor ,this fine diamond1 rose-cut and #ahf carats in weight1 is of the purest water and merits its title of #8,river of ,light4' ,it seems to have been captured by ,nadir ,shah at ,delhi1 and is now the largest diamond in the ,persian collection4 7#aj7 ,,shah ,this fine stone1 of the purest water1 was presented to the ,czar ,nicholas by the ,persian prince ,chosroes1 younger son of ,abbas ,mirza1 in #ahdc4 ,at that time it still retained three cleavage faces which were engraved with the names of three ,persian sovereigns1 and weighed #ie carats4 ,it was1 however1 subsequently re-cut with the loss of #i carats1 and the engraving has disappeared in the process4 7#aa7 ,,akbar ,,shah1 ,,or ,,jehan ,,ghir ,,shah ,once the property of the great ,mogul1 ,akbar1 this diamond was engraved on two faces with ,arabic inscriptions by the instructions of his successor1 ,jehan4 ,it disappeared1 but turned up again in ,turkey under the name of #8,shepherd's ,stone'2 it still retained its original inscriptions and was thereby recognized4 ,in #ahff it was re-cut1 the weight being reduced from #aaf to #ga carats1 and the inscriptions destroyed4 ,the stone was sold to the ,gaekwar of ,baroda for #c lakhs of rupees 7about #bc1ccc74 7#ab7 ,,polar ,,star ,a beautiful1 brilliant-cut stone1 weighing #dj carats1 which is known by this name1 is in the ,russian regalia4 7#ac7 ,,nassak ,the ,nassak diamond1 which weighed #hi carats1 formed part of the ,deccan booty1 and was put up to auction in ,london in ,july #ahcg4 ,it was purchased by ,emanuel1 a ,london jeweller1 who for #gbjj shortly afterwards sold it to the ,duke of ,westminster1 in whose family it still remains4 ,it was originally pear-shaped1 but was re-cut to a triangular form with a reduction in weight to #gh carats4 7#ad7 ,,napoleon ,this diamond was purchased by ,napoleon ,buonaparte for #hjjj1 and worn by him at his wedding with ,josephine ,beauharnais in #agif4 7#ae7 ,,cumberland ,this stone1 which weighs #cb carats1 was purchased by the city of ,london for #aj1jjj and presented to the ,duke of ,cumberland after the battle of ,culloden2 it is now in the possession of the ,duke of ,brunswick4 7#af7 ,,pigott ,a fine ,indian stone1 weighing #dg carats1 this diamond was brought to ,england by ,lord ,pigott in #agge and sold for #cj1jjj4 ,it came into the possession of ,ali ,pacha1 ,viceroy of ,egypt1 and was by his orders destroyed at his death4 7#ag7 ,,eug,,nie ,this fine stone1 weighing #ea carats1 was given by the ,czarina ,catherine ,,ii of ,russia to her favourite1 ,potemkin4 ,it was purchased by ,napoleon ,,iii as a bridal gift for his bride1 and on his downfall was bought by the ,gaekwar of ,baroda4 7#ah7 ,,white ,,saxon ,square in contour1 measuring #a-a_/ab in4 7#bh mm471 and weighing #dh carats1 this stone was purchased by ,augustus the ,strong for a million thalers 7about #aej1jjj74 7#ai7 ,,pacha ,,of ,,egypt ,this #dj-;carat brilliant was purchased by ,ibrahim1 ,viceroy of ,egypt1 for #bh1jjj4 7#bj7 ,,star ,,of ,,este ,though a comparatively small stone1 in weight #be carats1 it is noted for its perfection of form and quality4 ,it belongs to the ,archduke ,franz ,ferdinand of ,austrian-,este1 eldest son of the ,archduke ,karl ,ludwig4 7#ba7 ,,tuscany1 ,,or ,,austrian ,,yellow ,the beauty of this large stone1 #acc carats in weight1 is marred by the tinge of yellow1 which is sufficiently pronounced to impair its brilliancy2 it is a double rose in form4 ,at one time the property of the ,grand ,dukes of ,tuscany1 it is now in the possession of the ,emperor of ,austria4 ,king mentions a tale that it was bought at a curiosity stall in ,florence for an insignificant sum1 the stone being supposed to be only yellow quartz4 7#bb7 ,,star ,,of ,,the ,,south ,this1 the largest of the ,brazilian diamonds1 was discovered at the mines of ,bagagem in ,july #ahec4 ,perfectly transparent and without tint1 it was dodecahedral in shape and weighed #bed carats1 and was sold in the rough for #dj1jjj4 ,it was cut as a perfect brilliant1 being reduced in weight to #abe carats4 7#bc7 ,,english ,,dresden ,this beautiful stone1 which weighed #aai carats in the rough1 was found at the ,bagagem mines1 in ,brazil1 in #aheg1 and came into the possession of ,mr4 ,e4 ,dresden4 ,it was cut as a long1 egg-shaped brilliant1 weighing #gf carats4 7#bd7 ,,star ,,of ,,south ,,africa ,the first considerable stone to be found in ,south ,africa1 it was discovered at the ,vaal ,river diggings in #ahfi1 and weighed #hc carats in the rough4 ,it was cut to a triangular brilliant of #df carats4 ,it was finally purchased by the ,countess of ,dudley for #be1jjj4 7#be7 ,,stewart ,this large diamond1 weighing in the rough #bhh carats1 was found at the ,vaal ,river diggings in #ahgb1 and was first sold for #fjjj and shortly afterwards for #ijjj2 it was reduced on cutting to #abj carats4 ,like many ,south ,african stones1 it has a faint yellowish tinge4 7#bf7 ,,porter-,,rhodes ,this blue-white stone1 which weighed #aej carats1 was found in a claim belonging to ,mr4 ,porter-,rhodes in the ,kimberley mine in ,february #ahhj4 7#bg7 ,,imperial1 ,,victoria1 ,,or ,,great ,,white ,this large diamond weighed as much as #deg carats in the rough1 and #ahj when cut2 it is quite colourless4 ,it was brought to ,europe in #ahhd1 and was eventually sold to the ,nizam of ,hyderabad for #bj1jjj4 7#bh7 ,,de ,,beers ,a pale yellowish stone1 weighing #dbh carats1 was found in the ,de ,beers mine in #ahhh4 ,it was cut to a brilliant weighing #bbh carats1 and was sold to an ,indian prince4 ,a still larger stone of similar tinge1 weighing #ejc carats1 was discovered in #ahif1 and among other large stones supplied by the same mine may be mentioned one of #cjb carats found in #ahhd1 and another of #dji carats found in early years4 7#bi7 ,,excelsior ,this1 which prior to the discovery of the #8,cullinan1' was by far the largest ,south ,african stone1 was found in the ,jagersfontein mine on ,june #cj1 #ahic2 bluish-white in tint1 it weighed #ifi carats4 ,from it were cut twenty-one brilliants1 the larger stones weighing #fg1 #de-ac_/af1 #de-aa_/af1 #ci-c_/af1 #cd1 #bg1 #be1 #bc-aa_/af1 #af-aa_/cb1 #ac carats respectively1 and the total weight of the cut stones amounting to #cfd-c_/cb carats4 7#cj7 ,,jubilee ,another large stone was discovered in the ,jagersfontein mine in #ahie4 ,it weighed #fcd carats in the rough1 and from it was obtained a splendid1 faultless brilliant weighing #bci carats4 ,it was shown at the ,paris ,exhibition of #aijj4 7#ca7 ,,star ,,of ,,africa1 ,,or ,,cullinan ,illustration3 ,,fig4 #fg4--;,cullinan ,no4 #a4 ,all diamonds pale into insignificance when compared with the colossal stone that came to light at the ,premier mine near ,pretoria in the ,transvaal on ,january #be1 #aije4 ,it was first called the #8,cullinan' after ,sir ,t4 ,m4 ,cullinan1 chairman of the ,premier ,diamond ,mine 7,transvaal7 ,company1 but has recently1 by desire of ,king ,george ,v1 received the name #8,star of ,africa4' ,the rough stone weighed #fbab grams or #cjbe carats 7about #a lb472 it displayed three natural faces 7,plate ,,xxv7 and one large cleavage face1 and its shape suggested that it was a portion of an enormous stone more than double its size2 it was transparent1 colourless1 and had only one small flaw near the surface4 ,this magnificent diamond was purchased by the ,transvaal ,government for #aej1jjj1 and presented to ,king ,edward ,,vii on his birthday1 ,november #i1 #aijg4 ,illustration3 ,,plate ,,xxv ,,cullinan ,,diamond 7,natural size7 ,illustration3 ,,fig4 #fh4--;,cullinan ,no4 #b4 ,the ,cullinan was entrusted to the famous firm1 ,messrs4 ,i4 ,j4 ,asscher ,co41 of ,amsterdam1 for cutting on ,january #bc1 #aijh1 just three years after its discovery4 ,on ,february #aj it was cleaved into two parts1 weighing respectively #aigg and #ajdj carats1 from which the two largest stones have been cut1 one being a pendeloque or drop brilliant in shape 7,fig4 #fg7 and weighing #eaf carats1 and the other a square brilliant 7,fig4 #fh7 weighing #cji-c_/af carats4 ,the first has been placed in the sceptre1 and the second in the crown of the regalia4 ,besides these there are a pendeloque weighing #ib carats1 a square-shaped brilliant #fb1 a heart-shaped stone #ah1 two marquises #h-i_/af and #aa1 an oblong stone #f1 a pendeloque #d-i_/cb1 and #if small brilliants weighing together #g2 the total weight of the cut stones amounts to #ajcf-e_/cb carats4 ,the largest stone has #gd and the second #ff facets4 ,the work was completed and the stones handed to ,king ,edward in ,november #aijh4 ,although the ,premier mine has yielded no worthy compeer of the ,cullinan1 it can1 nevertheless1 boast of a considerable number of large stones which but for comparison with that giant would be thought remarkable for their size1 no fewer than seven of them having weights of over #cjj carats1 viz4 #eaa1 #dhg1 #deh1 #cia1 #cgc1 #cdh1 and #ccd carats4 7#cb7 ,,star ,,of ,,minas ,this large diamond1 which was found in #aiaa at the ,bagagem mines1 ,minas ,geraes1 ,brazil1 had the shape of a dome with a flat base1 and weighed in the rough #cehge grams 7#agd carats74 ,the large stone called the #8,braganza1' in the ,portuguese regalia1 which is supposed to be a diamond1 is probably a white topaz2 it weighs #afhj carats4 ,the ,mattan stone1 pear-shaped and weighing #cfg carats1 which was found in the ,landak mines near the west coast of ,borneo in #aghg1 is suspected to be quartz4 ,,coloured ,,diamonds 7#a7 ,,hope ,illustration3 ,,fig4 #fi4--;,hope4 ,the largest of coloured diamonds1 the ,hope1 weighs #dd carats1 and has a steely- or greenish-blue1 and not the royal-blue colour of the glass models supposed to represent it4 ,it is believed to be a portion of a drop-form stone 7d'un beau violet7 which was said to have been found at the ,kollur mines1 and was secured by ,tavernier in ,india in #afdb and sold by him to ,louis ,,xiv in #affh2 it then weighed #fg carats4 ,this stone was stolen with the remainder of the ,french regalia in #agib and never recovered4 ,in #ahcj the present stone 7,fig4 #fi7 was offered for sale by ,eliason1 a ,london dealer1 and was purchased for #ah1jjj by ,thomas ,philip ,hope1 a wealthy banker and a keen collector of gems4 ,probably the apex of the original stone had been cut off1 reducing it to a nearly square stone4 ,the slight want of symmetry of the present stone lends confirmation to this view1 and two other blue stones are known1 which1 together with the ,hope1 make up the weight of the original stone4 ,at the sale of the ,hope collection at ,christie's in #ahfg the blue diamond went to ,america4 ,in #aijh the owner disposed of it to ,habib ,bey for the enormous sum of #hj1jjj4 ,it was put up to auction in ,paris in #aiji1 and bought by ,rosenau1 the ,paris diamond merchant1 for the comparatively small sum of #djj1jjj francs 7about #af1jjj71 and was sold in ,january #aiaa to ,mr4 ,edward ,m',lean for #fj1jjj4 ,the stone is supposed to bring ill-luck in its train1 and its history has been liberally embellished with fable to establish the saying4 7#b7 ,,dresden ,a beautiful apple-green diamond1 faultless1 and of the purest water1 is contained in the famous ,green ,vaults of ,dresden4 ,it weighs #dj carats1 and was purchased by ,augustus the ,strong in #agdc for #fj1jjj thalers 7about #ijjj74 7#c7 ,,paul ,i ,a fine ruby-red diamond1 weighing #aj carats1 is included among the ,russian crown jewels4 7#d7 ,,tiffany ,the lovely orange brilliant1 weighing #abe carats1 which is in the possession of ,messrs4 ,tiffany ,co41 the well-known jewellers of ,new ,york1 was discovered in the ,kimberley mine in #ahgh4 ,,chapter ,,xix ,,corundum 7,sapphire1 ,ruby7 ,ranking in hardness second to diamond alone1 the species known to science as corundum and widely familiar by the names of its varieties1 sapphire and ruby1 holds a pre-eminent position among coloured gem-stones4 ,the barbaric splendour of ruby 7,plate ,i1 ,fig4 #ac7 and the glorious hue of sapphire 7,plate ,i1 ,fig4 #aa7 are unsurpassed1 and it is remarkable that the same species should boast such different1 but equally magnificent1 tints4 ,they1 however1 by no means exhaust the resources of this variegated species4 ,fine yellow stones 7,plate ,i1 ,fig4 #ab71 which compare with topaz in colour and are its superior in hardness1 and brilliant colourless stones1 which are unfortunately deficient in #8fire' and cannot therefore approach diamond1 are to be met with1 besides others of less attractive hues1 purple1 and yellowish1 bluish1 and other shades of green4 ,want of homogeneity in the coloration of corundum is a frequent phenomenon2 thus1 the purple stones on close examination are found to be composed of alternate blue and red layers1 and stones showing patches of yellow and blue colour are common4 ,owing to the peculiarity of their interior arrangement certain stones display when cut en cabochon a vivid six-rayed star of light 7,plate ,i1 ,fig4 #ae74 ,sapphire and ruby share with diamond1 pearl1 and emerald the first rank in jewellery4 ,they are popular stones1 especially in rings2 their comparative rarity in large sizes1 apart from the question of expense1 prevents their use in the bigger articles of jewellery4 ,the front of the stones is usually brilliant-cut and the back step-cut1 but ,indian lapidaries often prefer to cover the stone with a large number of triangular facets1 especially if the stone be flawed2 star-stones are cut more or less steeply en cabochon4 ,in composition corundum is alumina1 oxide of aluminium1 corresponding to the formula ,al#b,o#c1 but it usually contains in addition small quantities1 rarely more than #a per cent41 of ferric oxide1 chromic oxide1 and perhaps other metallic oxides4 ,when pure1 it is colourless2 the splendid tints which are its glory have their origin in the minute traces of the other oxides present4 ,no doubt chromic oxide is the cause of the ruddy hue of ruby1 since it is possible1 as explained above 7p4 #aag71 closely to imitate the ruby tint by this means1 but nothing approaching so large a percentage as #b has been detected in a natural stone4 ,the blue colour of sapphire may be due to titanic oxide1 and ferric oxide may be responsible for the yellow hue of the #8oriental topaz1' as the yellow corundum is termed4 ,sapphires1 when of considerable size1 are rarely uniform in tint throughout the stone4 ,alternations of blue and red zones1 giving rise to an apparent purple or violet tint1 and the conjunction of patches of blue and yellow are common4 ,perfectly colourless stones are less common1 a slight bluish tinge being usually noticeable1 but they are not in much demand because1 on account of their lack of #8fire1' they are of little interest when cut4 ,the tint of the red stones varies considerably in depth2 jewellers term them1 when pale1 pink sapphires1 but1 of course1 no sharp distinction can be drawn between them and rubies4 ,the most highly prized tint is the so-called pigeon's blood1 a shade of red slightly inclined to purple4 ,the prices for ruby of good colour run from about #bes4 a carat for small stones to between #fj and #hj a carat for large stones1 and still higher for exceptional rubies4 ,the taste in sapphires has changed of recent times4 ,formerly the deep blue was most in demand1 but now the lighter shade1 that resembling the colour of corn-flower1 is preferred1 because it retains a good colour in artificial light4 ,large sapphires are more plentiful than large rubies1 and prices run lower2 even for large perfect stones the rate does not exceed #cj a carat4 ,large and uniform #8oriental topazes' are comparatively common1 and realize moderate prices1 about #bs4 to #cjs4 a carat according to quality and size4 ,green sapphires are abundant from ,australia1 but their tint1 a kind of deep sage-green1 is not very pleasing4 ,brown stones with a silkiness of structure are also known4 ,the name of the species comes through the ,french corindon from an old ,hindu word1 korund1 of unknown significance1 and arose from the circumstance that the stones which first found their way to ,europe came from ,india4 ,at the present day the word corundum is applied in commerce to the opaque stones used for abrasive purposes1 to distinguish the purer material from emery1 which is corundum mixed with magnetite and other heavy stones of lower hardness4 ,the origin of the word sapphire1 which means blue1 has been discussed in an earlier chapter 7p4 #aaj74 ,jewellers use it in a general sense for all corundum except ruby4 ,ruby comes from the ,latin ruber1 red4 ,the prefix #8oriental' 7p4 #aaa7 is often used to distinguish varieties of corundum1 since it is the hardest of ordinary coloured stones and the finest gem-stones in early days reached ,europe by way of the ,east4 ,corundum crystallizes either in six-sided prisms terminated by flat faces 7,plate ,i1 ,fig4 #aj71 which are often triangularly marked1 or with twelve inclined faces1 six above and six below1 meeting in a girdle 7,plate ,i1 ,fig4 #ad74 ,ruby favours the former and the other varieties the latter type4 ,a fine crystal of ruby--the #8,edwardes1' so named by the donor1 ,john ,ruskin1 after ,sir ,herbert ,edwardes--which weighs #cce grams 7#afc carats71 is exhibited in the ,mineral ,gallery of the ,british ,museum 7,natural ,history71 and is tilted in such a way that the light from a neighbouring window falls on the large basal face1 and reveals the interesting markings that nature has engraved on it4 ,from its type of symmetry corundum is doubly refractive with a direction of single refraction running parallel to the edge of the prism4 ,owing to the relative purity of the chemical composition the refractive indices are very constant2 the ordinary index ranges from #agff to #aggd and the extraordinary index from #ageg to #agfe1 the double refraction remaining always the same1 #jjji4 ,the amount of colour-dispersion is small1 and therefore colourless corundum displays very little #8fire4' ,the difference between the indices for red and blue light is1 however1 sufficiently great that the base of a ruby may be left relatively thicker than that of a sapphire to secure an equally satisfactory effect 7cf4 p4 #ih7--;a point of some importance to the lapidary1 since stones are sold by weight and it is his object to keep the weight as great as possible4 ,when a corundum is tested on the refractometer in white light a wide spectrum deliminates the two portions of the field because of the smallness of the colour-dispersion 7cf p4 #be74 ,the dichroism of both ruby and sapphire is marked1 the twin colours given by the former being red and purplish-red1 and by the latter blue and yellowish-blue1 the second colour in each instance corresponding to the extraordinary ray4 ,tests with the dichroscope easily separate ruby and sapphire from any other red or blue stone4 ,this character has an important bearing on the proper mode of cutting the stones4 ,the ugly yellowish tint given by the extraordinary ray of sapphire should be avoided by cutting the stone with its table-facet at right angles to the prism edge1 which is the direction of single refraction4 ,whether a ruby should be treated in the same way is a moot point4 ,no doubt if the colour is deep1 it is the best plan1 because the amount of absorption of light is thereby sensibly reduced1 but otherwise the delightful nuances distinguishing ruby are best secured by cutting the table-facet parallel to the direction of single refraction4 ,yellow corundum also shows distinct dichroism1 but by a variation more of the depth than of the tint of the colour2 the phenomenon is faint compared with the dichroic effect of a yellow chrysoberyl4 ,the specific gravity also is very constant1 varying only from #cie to #daj2 sapphire is on the whole lighter than ruby4 ,corundum has the symbol #i on ,mohs's scale1 but though coming next to diamond it is a very poor second 7cf4 p4 #gi74 ,as is usually the case1 the application of heat tends to lighten the colour of the stones3 those of a pale violet or a yellow colour lose the tint entirely1 and the deep violet stones turn a lovely rose colour4 ,on the other hand the action of radium has1 as was shown by ,bordas1 an intensifying action on the colour1 and even develops it in a colourless stone4 ,from the latter reaction it may be inferred that often in an apparently colourless stone two or more selective influences are at work which ordinarily neutralize one another1 but1 being unequally stimulated by the action of radium1 they thereupon give rise to colour4 ,the stellate appearance of asterias or star-stones--star-ruby and star-sapphire--results from the regular arrangement either of numerous small channels or of twin-lamell in the stone parallel to the six sides of the prisms2 light is reflected from the interior in the form of a six-rayed star 7p4 #ch74 ,some stones from ,siam possess a markedly fibrous or silky structure4 ,the synthetical manufacture of ruby1 sapphire1 and other varieties of corundum has already been described 7p4 #aaf74 ,besides its use in jewellery corundum is on account of its hardness of great service for many other purposes4 ,small fragments are extensively employed for the bearing parts of the movements of watches1 and both the opaque corundum and the impure kind known as emery are in general use for grinding and polishing softer stones1 and steel and other metal-work4 ,the world's supply of fine rubies is drawn almost entirely from the famous ruby mines near ,mogok1 situated about #ij miles 7#ade km47 in a north-easterly direction from ,mandalay in ,upper ,burma and at an elevation of about #djjj ft4 7#abjj m47 above sea-level4 ,it is from this district that the stones of the coveted carmine-red1 the so-called #8pigeon's blood1' colour are obtained4 ,the ruby occurs in a granular limestone or calcite in association with the spinel of nearly the same appearance--the #8balas-ruby1' oriental topaz 7yellow corundum71 tourmaline1 and occasionally sapphire4 ,some stones are found in the limestone on the sides of the hills1 but by far the largest quantity occur in the alluvial deposits1 both gravel and clay1 in the river-beds2 the ruby ground is locally known as #8byon4' ,the stones are as a rule quite small1 averaging only about four to the carat4 ,before the ,british annexation of the country in #ahhe the mines were a monopoly of the ,burmese sovereigns and were worked solely under royal licence4 ,they are known to be of great antiquity1 but otherwise their early history is a mystery4 ,it is said that an astute king secured the priceless territory in #aeig from the neighbouring ,chinese ,shans in exchange for a small and unimportant town on the ,irrawaddy2 if that be so1 he struck an excellent bargain4 ,the mines were allotted to licensed miners1 twin-tsas 7eaters of the mine7 as they were called in the language of the country1 who not only paid for the privilege1 but were compelled to hand over to the king all stones above a certain weight4 ,as might be anticipated this injunction caused considerable trouble1 and the royal monopolists constantly suspected the miners of evading the regulation by breaking up stones of exceptional size2 from subsequent experience1 it is probable that large stones were in reality seldom found4 ,since #ahhg the mines have been worked by arrangement with the ,government of ,india by the ,ruby ,mines1 ,ltd41 an ,english company4 ,its career has been far from prosperous1 but during recent years1 in consequence of the improved methods of working the mines and of the more generous terms afterwards accorded by the ,government1 greater success has been experienced2 the future is1 however1 to some extent clouded by the advent of the synthetical stone1 which has even made its way out to the ,east4 ,large rubies are far from common1 and such as were discovered in the old days were jealously hoarded by the ,burmese sovereigns4 ,according to ,streeter the finest that ever came to ,europe were a pair brought over in #ahge1 at a time when the ,burmese king was pressed for money4 ,one1 rich in colour1 was originally cushion-shaped and weighed #cg carats2 the other was a blunt drop in form and weighed #dg carats4 ,both were cut in ,london1 the former being reduced to #cb-e_/af carats and the latter to #ch-i_/af carats1 and were sold for #aj1jjj and #bj1jjj respectively4 ,a colossal stone1 weighing #djj carats1 is reported to have been found in ,burma2 it was broken into three pieces1 of which two were cut and resulted in stones weighing #gj and #de carats respectively1 and the third was sold uncut in ,calcutta for #g lakhs of rupees 7#df1ffg74 ,the finder of another large stone broke it into two parts1 which after cutting weighed #ih and #gd carats respectively2 he attempted in vain to evade the royal acquisitiveness1 by giving up the larger stone to the king and concealing the other4 ,a fine stone1 known by the formidable appellation of #8,gnaga ,boh' 7,dragon ,lord71 weighed #dd carats in the rough and #bj carats after cutting4 ,since the mines were taken over by the ,ruby ,mines1 ,ltd41 a few large stones have been discovered4 ,a beautiful ruby was found in the ,tagoungnandaing ,valley1 and weighed #ah carats in the rough and #aa carats after cutting2 perfectly clear and of splendid colour1 it was sold for #gjjj1 but is now valued at #aj1jjj4 ,another1 weighing #gg carats in the rough1 was found in #ahii1 and was sold in ,india in #aijd for #d lakhs of rupees 7#bf1ffg74 ,a stone1 weighing #di carats1 was discovered in #ahhg1 and an enormous one1 weighing as much as #cjd carats1 in #ahij4 ,the ruby1 as large as a pigeon's egg1 which is amongst the ,russian regalia was presented in #aggg to the ,czarina ,catherine by ,gustav ,,iii of ,sweden when on a visit to ,st4 ,petersburg4 ,the large red stone in the ,english regalia which was supposed to be a ruby is a spinel 7cf p4 #bjf74 ,comparatively uncommon as sapphires are in the ,burma mines a faultless stone1 weighing as much as #gi carats1 has been discovered there4 ,good rubies1 mostly darker in colour than the ,burmese stones1 are found in considerable quantity near ,bangkok in ,siam1 ,chantabun being the centre of the trade1 where1 just as in ,burma1 they are intimately associated with the red spinel4 ,because of the difference in tint and the consequent difference in price1 jewellers draw a distinction between ,burma and ,siam rubies2 but that1 of course1 does not signify any specific difference between them4 ,siam is1 however1 most distinguished as the original home of splendid sapphires4 ,the district of ,bo ,pie ,rin in ,battambang produces1 indeed1 more than half the world's supply of sapphires4 ,in the ,hills of ,precious ,stones1 such being the meaning of the native name for the locality1 a number of green corundums are found4 ,siam also produces brown stones characterized by a peculiar silkiness of structure4 ,rubies are found in ,afghanistan at the ,amir's mines near ,kabul and also to the north of the lapis lazuli mines in ,badakshan4 ,the conditions in ,ceylon are precisely the converse of those obtaining in ,burma2 sapphire is plentiful and ruby rare in the island4 ,they are found in different rocks1 sapphire occurring with garnet in gneiss1 and ruby accompanying spinel in limestone1 but they come together in the resulting gravels1 the principal locality being the gem-district near ,ratnapura in the south of the island4 ,the largest uncut ruby discovered in ,ceylon weighed #db carats2 it had1 however1 a decided tinge of blue in it4 ,ceylon is also noted for the magnificent yellow corundum1 #8oriental topaz1' or1 as it is locally called1 #8king topaz1' which it produces4 ,beautiful sapphires occur in various parts of ,india1 but particularly in the ,zanskar range of the north-western ,himalayas in the state of ,kashmir1 where they are associated with brown tourmaline4 ,probably most of the large sapphires known have emanated from ,india4 ,by far the most gigantic ever reported is one1 weighing #iea carats1 said to have been seen in #ahbg in the treasury of the ,king of ,ava4 ,the collection at the ,jardin des ,plantes contains two splendid rough specimens2 one1 known as the #8,rospoli1' is quite flawless and weighs #acb-a_/af carats1 and the other is #b inches in length and #a inches in thickness4 ,the ,duke of ,devonshire possesses a fine cut stone1 weighing #ajj carats1 which is brilliant-cut above and step-cut below the girdle4 ,an image of ,buddha1 which is cut out of a single sapphire1 is exhibited1 mounted on a gold pin1 in the ,mineral ,gallery of the ,british ,museum 7,natural ,history74 ,for some years past a large quantity of sapphires have come into the market from ,montana1 ,u4,s4,a41 especially from the gem-district about twelve miles west of ,helena4 ,the commonest colour is a bluish green1 generally pale1 but blue1 green1 yellow and occasionally red stones are also found2 they are characterized by their almost metallic lustre4 ,with them are associated gold1 colourless topaz1 kyanite1 and a beautiful red garnet which is found in grains and usually mistaken for ruby4 ,rubies are also found in limestone at ,cowee ,creek1 ,north ,carolina4 ,blue and red corundum1 of rather poor quality1 has come from the ,sanarka ,river1 near ,troitsk1 and from ,miask1 in the ,government of ,orenburg1 ,russia1 and similar stones have been known at ,campolongo1 ,st4 ,gothard1 ,switzerland4 ,the prolific gem-district near ,anakie1 ,queensland1 supplies examples of every known variety of corundum except ruby2 blue1 green1 yellow1 and parti-coloured stones1 and also star-stones1 are plentiful4 ,leaf-green corundum is known father south1 in ,victoria4 ,the ,australian sapphire is too dark to be of much value4 ,small rubies and sapphires are found in the gem-gravels near the ,somabula ,forest1 ,rhodesia4 ,,chapter ,,xx ,,beryl 7,emerald1 ,aquamarine1 ,morganite7 ,the species to be considered in this chapter includes the varieties emerald and aquamarine1 as well as what jewellers understand by beryl4 ,it has many incontestable claims on the attention of all lovers of the beautiful in precious stones4 ,the peerless emerald 7,plate ,i1 ,fig4 #e71 which in its verdant beauty recalls the exquisite lawns that grace the courts and quadrangles of our older seats of learning1 ranks to-day as the most costly of jewels4 ,its sister stone1 the lovely aquamarine 7,plate ,i1 ,fig4 #d71 which seems to have come direct from some mermaid's treasure-house in the depths of a summer sea1 has charms not to be denied4 ,pliny1 speaking of this species1 truly says1 8,there is not a colour more pleasing to the eye02 yet he knew only the comparatively inferior stones from ,egypt1 and possibly from the ,ural ,mountains4 ,emeralds are favourite ring-stones1 and would1 no doubt1 be equally coveted for larger articles of jewellery did not the excessive cost forbid1 and nothing could be more attractive for a central stone than a choice aquamarine of deep blue-green hue4 ,emeralds are usually step-cut1 though ,indian lapidaries often favour the en cabochon form2 aquamarines1 on the other hand1 are brilliant-cut in front and step-cut at the back4 ,beryl1 to use the name by which the species is known to science1 is essentially a silicate of aluminium and beryllium corresponding to the formula1 ,be#c;,al#b7,si,o#c7f4 ,the beryllia is often partially replaced by small amounts of the alkaline earths1 caesia1 potash1 soda1 and lithia1 varying from about #a per cent4 in beryl from ,mesa ,grande to nearly #e in that from ,pala and ,madagascar1 and over #f1 of which #cf is caesia1 in beryl from ,hebron1 ,maine2 also1 as usual1 chromic and ferric oxides take the place of a little alumina2 from #a to #b per cent4 of water has been found in emerald4 ,the element beryllium was1 as its name suggests1 first discovered in a specimen of this species1 the discovery being made in #agih by the chemist ,vauquelin2 it is also known as glucinum in allusion to the sweet taste of its salts4 ,when pure1 beryl is colourless1 but it is rarely1 if ever1 free from a tinge of blue or green4 ,the colour is usually some shade of green--grass-green1 of that characteristic tint which is in consequence known as emerald-green1 or blue-green1 yellowish green 7,plate ,i1 ,fig4 #f71 and sometimes yellow1 pink1 and rose-red4 ,the peculiar colour of emerald is supposed to be caused by chromic oxide1 small quantities of which have been detected in it by chemical analysis2 moreover1 experiment shows that glass containing the same percentage amount of chromic oxide assumes the same splendid hue4 ,emerald1 on being heated1 loses water1 but retains its colour unimpaired1 which cannot therefore be due1 as has been suggested1 to organic matter4 ,the term aquamarine is applied to the deep sea-green and blue-green stones1 and jewellers restrict the term beryl to paler shades and generally other colours1 such as yellow1 golden1 and pink1 but ,kunz has recently proposed the name morganite to distinguish the beautiful rose beryl such as is found in ,madagascar4 ,the varying shades of aquamarine are due to the influence of the alkaline earths modified by the presence of ferric oxide or chromic oxide2 the beautiful blushing hue of morganite is no doubt caused by lithia4 ,illustration3 ,,fig4 #gj4--;,emerald ,crystal4 ,the name of the species is derived from the ,greek 1 an ancient word1 the meaning of which has been lost in the mists of time4 ,the ,greek word denoted the same species in part as that now understood by the name4 ,emerald is derived from a ,persian word which appeared in ,greek as 1 and in ,latin as smaragdus2 it originally denoted chrysocolla1 or similar green stone1 but was transferred upon the introduction of the deep-green beryl from ,upper ,egypt4 ,the name aquamarine was suggested by ,pliny's exceedingly happy description of the stones 8which imitate the greenness of the clear sea10 although it was not actually used by him4 ,that emerald and beryl were one species was suspected by ,pliny1 but the identity was not definitely established till about a century ago4 ,morganite is named after ,john ,pierpont ,morgan4 ,the natural crystals have the form of a six-sided prism1 and in the case of emerald 7,fig4 #gj1 and ,plate ,i1 ,fig4 #h7 invariably1 if whole1 end in a single face at right angles to the length of the prism2 aquamarines have in addition a number of small inclined faces1 and stones from both ,russia and ,brazil often taper owing to the effects of corrosion4 ,the sixfold character of the crystalline symmetry necessarily entails that the double refraction1 which is small in amount1 #jjjf1 is uniaxial in character1 and1 since the ordinary is greater than the extraordinary refractive index1 it is negative in sign4 ,the values of the indices range between #aefg and #aeij1 and #aegb and #aeih respectively1 in the two cases1 the pink beryl possessing the highest values4 ,the dichroism is distinct in the ,south ,american emerald1 the twin colours being yellowish and bluish green1 but otherwise is rather faint4 ,the specific gravity varies between #bfi and #bgi1 and is therefore a little higher than that of quartz4 ,if1 therefore1 a beryl and a quartz be floating together in a tube containing a suitable heavy liquid1 the former will always be at a sensibly lower level 7cf4 ,fig4 #cb74 ,the hardness varies from #g to #h1 emerald being a little softer than the other varieties4 ,there is no cleavage1 but like most gem-stones beryl is very brittle1 and can easily be fractured4 ,stones rendered cloudy by fissures are termed #8mossy4' ,when heated before the blowpipe beryl is fusible with difficulty2 it resists the attack of hydrofluoric acid as well as of ordinary acids4 ,in all probability the whole of the emeralds known in ancient times came from the so-called ,cleopatra emerald mines in ,upper ,egypt4 ,for some reason they were abandoned1 and their position was so completely lost that in the ,middle ,ages it was maintained that emeralds had never been found in ,egypt at all1 but had come from ,america by way of the ,east4 ,all doubts were set at rest by the re-discovery of the mines early last century by ,cailliaud1 who had been sent by the ,viceroy of ,egypt to search for them4 ,they were1 however1 not much worked1 and after a few years were closed again1 and were re-opened only about ten years ago4 ,the principal mines are at ,jebel ,zabara and at ,jebel ,sikait in northern ,etbai1 about #aj miles 7#af km47 apart and distant about #ae miles 7#bd km47 from the ,red ,sea1 lying in the range of mountains that run for a long distance parallel to the west coast of the ,red ,sea and rise to over #ahjj feet 7#eej m47 above sea-level4 ,there are numerous signs of considerable1 but primitive1 workings at distinct periods4 ,both emeralds and beryls are found in micaceous and talcose schists4 ,the emeralds are not of very good quality1 being cloudy and rather light in colour4 ,finer emeralds have been found in a dark mica-schist1 together with other beryllium minerals1 chrysoberyl and phenakite1 and also topaz and tourmaline on the ,asiatic side of the ,ural ,mountains1 near the ,takowaja ,river1 which flows into the ,bolshoi ,reft ,river1 one of the larger tributaries of the ,pyschma ,river1 about fifty miles 7#hj km47 east of ,ekaterinburg1 a town which is chiefly concerned with the mining and cutting of gem-stones4 ,the mine was accidentally discovered by a peasant1 who noticed a few green stones at the foot of an uprooted tree in #ahcj4 ,two years later the mine was regularly worked1 and remained open for twenty years1 when it was closed4 ,it has recently been re-opened owing to the high rates obtaining for emeralds4 ,very large crystals have been produced here1 but in colour they are much inferior to the ,south ,american stones2 small ,siberian emeralds1 on the other hand1 are of better colour than small ,south ,american emeralds1 the latter being not so deep in tint4 ,emeralds have been found in a similar kind of schist at ,habachtal1 in the ,salzburg ,alps4 ,about thirty years ago well-formed green stones were discovered with hiddenite at ,stony ,point1 ,alexander ,county1 in ,north ,carolina1 but not much gem material has come to light4 ,the products of none of the mines that have just been mentioned can on the whole compare with the beautiful stones which have come from ,south ,america4 ,at the time when the ,spaniards grimly conquered ,peru and ruthlessly despoiled the country of the treasures which could be carried away1 immense numbers of emeralds--some of almost incredible size--were literally poured into ,spain1 and eventually found their way to other parts of ,europe4 ,these stones were known as ,spanish or ,peruvian emeralds1 but in all probability none of them were actually mined in ,peru4 ,perhaps the most extraordinary were the five choice stones which ,cortez presented to his bride1 the niece of the ,duke de ,bejar1 thereby mortally offending the ,queen1 who had desired them for herself1 and which were lost in #aebi when ,cortez was shipwrecked on his disastrous voyage to assist ,charles ,v at the siege of ,algiers4 ,all five stones had been worked to divers fantastic shapes4 ,one was cut like a bell with a fine pearl for a tongue1 and bore on the rim1 in ,spanish1 8,blessed is he who created thee40 ,a second was shaped like a rose1 and a third like a horn4 ,a fourth was fashioned like a fish1 with eyes of gold4 ,the fifth1 which was the most valuable and the most remarkable of all1 was hollowed out into the form of a cup1 and had a foot of gold2 its rim1 which was formed of the same precious metal1 was engraved with the words1 8,inter natos mulierum non surrexit major40 ,as soon as the ,spaniards had seized nearly all the emeralds that the natives had amassed in their temples or for personal adornment1 they devoted their attention to searching for the source of these marvels of nature1 and eventually in #aeeh they lighted by accident upon the mines in what is now the ,united ,states of ,colombia1 which have been worked almost continuously since that time4 ,since the natives1 who naturally resented the gross injustice with which they had been treated1 and penetrated the greed that prompted the actions of the ,spaniards1 hid all traces of the mines1 and refused to give any information as to their position1 it is possible that other emerald mines may yet be found4 ,the present mines are situated near the village of ,muzo1 about #ge miles 7#abj km47 north-north-west of ,bogota1 the capital of ,colombia4 ,the emeralds occur in calcite veins in a bituminous limestone of ,cretaceous age4 ,the ,spaniards formerly worked the mines by driving adits through the barren rock on the hillsides to the gem-bearing veins1 but at the present day the open cut method of working is employed4 ,a plentiful supply of water is available1 which is accumulated in reservoirs and allowed at the proper time to sweep the debris of barren rock away into the ,rio ,minero1 leaving the rock containing the emeralds exposed4 ,stones1 of good quality1 which are suited for cutting1 are locally known as canutillos1 inferior stones1 coarse or ill-shaped1 being called morallons4 ,emerald1 unlike some green stones1 retains its purity of colour in artificial light2 in fact1 to quote the words of ,pliny1 8,for neither sun nor shade1 nor yet the light of candle1 causeth to change and lose their lustre40 ,many are the superstitions that have been attached to it4 ,thus it was supposed to be good for the eyes1 and as ,pliny says1 8,besides1 there is not a gem or precious stone that so fully possesseth the eye1 and yet never contenteth it with satiety4 ,nay1 if the sight hath been wearied and dimmed by intentive poring upon anything else1 the beholding of this stone doth refresh and restore it again40 ,the idea that it was fatal to the eyesight of serpents appears in ,moore's lines-- 8,blinded like serpents when they gaze ,upon the emerald's virgin blaze40 ,the crystals occur attached to the limestone1 and are therefore never found doubly terminated4 ,the crystal form is very simple1 merely a hexagonal prism with a flat face at the one end at right angles to it4 ,they are invariably flawed1 so much so that a flawless emerald has passed into proverb as unattainable perfection4 ,the largest single crystal which is known to exist at the present day is in the possession of the ,duke of ,devonshire 7,fig4 #ga74 ,in section it is nearly a regular hexagon1 about #b inches 7#ea mm47 in diameter from side to side1 and the length is about the same2 its weight is #bgfgi grams 7#i oz4 ,av41 or #acdg carats74 ,it is of good colour1 but badly flawed4 ,it was given to the ,duke of ,devonshire by ,dom ,pedro of ,brazil1 and was exhibited at the ,great ,exhibition of #ahea4 ,a fine1 though much smaller crystal1 but of even better colour1 which weighs #cbb grams 7#aef carats71 and measures #a inch 7#bh mm47 in its widest cross-diameter1 and about the same in length1 was acquired with the ,allan-,greg collection by the ,british ,museum1 and is exhibited in the ,mineral ,gallery of the ,british ,museum 7,natural ,history74 ,the finest cut emerald is said to be one weighing #cj carats1 which belongs to the ,czar of ,russia4 ,a small1 but perfect and flawless1 faceted emerald1 which is set in a gold hoop1 is also in the ,british ,museum 7,natural ,history74 ,it is shown1 without the setting1 about actual size1 on ,plate ,i1 ,fig4 #e4 ,illustration3 ,,fig4 #ga4--;,duke of ,devonshire's ,emerald4 7,natural size47 ,the ever great demand and the essentially restricted supply have forced the cost of emeralds of good quality to a height that puts large stones beyond the reach of all but a privileged few who have purses deep enough4 ,the rate per carat may be anything from #ae upwards1 depending upon the purity of the colour and the freedom from flaws1 but it increases very rapidly with the size1 since flawless stones of more than #d carats or so in weight are among the rarest of jewels2 a perfect emerald of #d carats may easily fetch #afjj to #bjjj4 ,it seems anomalous to say that it has never been easier to procure fine stones than during recent years1 but the reason is that the high prices prevailing have tempted owners of old jewellery to realize their emeralds4 ,on the other hand1 pale emeralds are worth only a nominal sum4 ,the other varieties of beryl are much less rare1 and1 since they usually attain to more considerable1 and sometimes even colossal1 size1 far larger stones are obtainable4 ,an aquamarine1 particularly of good deep blue-green colour1 is a stone of great beauty1 and it possesses the merit of preserving its purity of tint in artificial light4 ,it is a favourite stone for pendants1 brooches1 and bracelets1 and all purposes for which a large blue or green stone is desired4 ,the varying tints are said to be due to the presence of iron in different percentages1 and possibly in different states of oxidation4 ,unlike emerald1 the other varieties are by no means so easily recognized by their colour4 ,blue aquamarines may easily be mistaken for topaz1 or vice versa1 and the yellow beryl closely resembles other yellow stones1 such as quartz1 topaz1 or tourmaline4 ,stones which are colourless or only slightly tinted command little more than the price of cutting1 but the price of blue-green stones rapidly advances with increasing depth of tint up to #b a carat4 ,the enormous cut aquamarine which is exhibited in the ,mineral ,gallery of the ,british ,museum 7,natural ,history71 affords some idea of the great size such stones reach2 a beautiful sea-green in colour1 it weighs #agie grams 7#hge carats71 and is table-cut with an oval contour4 ,the splendid six-sided columns which have been discovered in various parts of ,siberia are among the most striking specimens in any large mineral collection4 ,the neighbourhood of ,ekaterinburg in the ,urals is prolific in varieties of aquamarine2 especially at ,mursinka have fine stones been found1 in association with topaz1 amethyst1 and schorl1 the black tourmaline4 ,good stones also occur in conjunction with topaz at ,miask in the ,government of ,orenburg4 ,it is found in the gold-washings of the ,sanarka ,river1 in the ,southern ,urals1 but the stones are not fitted for service as gems4 ,magnificent blue-green and yellow aquamarines are associated with topaz and smoky quartz in the granite of the ,adun-,tschilon ,mountains1 near ,nertschinsk1 ,transbaikal4 ,stones have also been found at the ,urulga ,river in ,siberia4 ,most of the bluish-green aquamarines which come into the market at the present time have originated in ,brazil1 particularly in ,minas ,novas1 ,minas ,geraes1 where clear1 transparent stones1 of pleasing colour1 in various shades1 are found in the utmost profusion2 beautiful yellow stones also occur at the ,bahia mines4 ,aquamarine was obtained in very early times in ,coimbatore ,district1 ,madras1 ,india1 and yellow beryl comes from ,ceylon4 ,fine blue crystals occur in the granite of the ,mourne ,mountains1 ,ireland1 but they are not clear enough for cutting purposes2 similar stones are found also at ,limoges1 ,haute ,vienne1 ,france4 ,aquamarines of various hues abound in several places in the ,united ,states1 among the principal localities being ,stoneham in ,maine1 ,haddam in ,connecticut1 and ,pala and ,mesa ,grande in ,san ,diego ,county1 ,california4 ,the last-named state is remarkable for the numerous stones of varying depth of salmon-pink that have been found there4 ,it is1 however1 surpassed by ,madagascar1 which has recently produced splendid stones of perfect rose-red tint and of the finest gem quality1 some of them being nearly #ajj carats in weight4 ,these stones1 which have been assigned a special name1 morganite 7cf4 supra71 are associated with tourmaline and kunzite4 ,pink and yellow beryls and deep blue-green aquamarines occur in the island in quantity4 ,the pink beryls from ,california are generally pale or have a pronounced salmon tint1 and seldom approach the real rose-red colour of morganite2 one magnificent rose-red crystal1 weighing nearly #i lb4 7#dje kg471 has1 however1 been recently discovered in ,san ,diego ,county1 ,california1 and is now in the ,british ,museum 7,natural ,history74 ,blue-green beryl1 varying in tint from almost colourless to an emerald-green1 occurs with tin-stone and topaz about #i miles 7#ad km47 north-east of ,emmaville in ,new ,south ,wales1 ,australia4 ,probably the largest and finest aquamarine crystal ever seen was one found by a miner on ,march #bh1 #aiaj1 at a depth of #ae ft4 7#e m47 in a pegmatite vein at ,marambaya1 near ,arassuahy1 on the ,jequitinhonha ,river1 ,minas ,geraes1 ,brazil4 ,it was greenish blue in colour1 and a slightly irregular hexagonal prism1 with a flat face at each end1 in form2 it measured #ai in4 7#dhe cm47 in length and #af in4 7#da cm47 in diameter1 and weighed #bdc lb4 7#aaje kg472 and its transparency was so perfect that it could be seen through from end to end 7,plate ,,xxvi74 ,the crystal was transported to ,bahia1 and sold for #be1jjj 7#eacc74 ,illustration3 ,,plate ,,xxvi ,,large ,,aquamarine ,,crystal 7one-sixth natural size71 ,,found ,,at ,,marambaya1 ,,minas ,,geraes1 ,,brazil ,,part ,,ii--,,section ,b ,,semi-,,precious ,,stones ,,chapter ,,xxi ,,topaz ,topaz is the most popular yellow stone in jewellery1 and often forms the principal stone in brooches or pendants1 especially in old-fashioned articles4 ,it is a general idea that all yellow stones are topazes1 and all topazes are yellow2 but neither statement is correct4 ,a very large number of yellow stones that masquerade as topaz are really the yellow quartz known as citrine4 ,the latter is1 indeed1 almost universally called by jewellers topaz1 the qualification #8,brazilian' being used by them to distinguish the true topaz4 ,many species besides those mentioned yield yellow stones4 ,thus corundum includes the beautiful #8oriental topaz' or yellow sapphire1 and yellow tourmalines are occasionally met with2 the yellow chrysoberyl always has a greenish tinge4 ,topaz is generally brilliant-cut in front and step-cut at the back1 and the table facet is sometimes rounded1 but the colourless stones are often cut as small brilliants2 it takes an excellent and dazzling polish4 ,topaz is a silicate of aluminium corresponding to the formula ,al7,f1,,oh7#b,si,o#d1 which was established in #ahid by ,penfield and ,minor as the result of careful research4 ,contrary to the general idea1 topaz is usually colourless or very pale in tint4 ,yellow hues of different degrees1 from pale to a rich sherry tint 7,plate ,i1 ,fig4 #i71 are common1 and pure pale blue 7,plate ,i1 ,fig4 #g7 and pale green stones1 which often pass as aquamarine1 are far from rare4 ,natural1 red and pink1 stones are very seldom to be met with4 ,it is1 however1 a peculiarity of the brownish-yellow stones from ,brazil that the colour is altered by heating to a lovely rose-pink4 ,curiously1 the tint is not apparent when the stone is hot1 but develops as it cools to a normal temperature2 the colour seems to be permanent4 ,such stones are common in modern jewellery4 ,although the change in colour is accompanied by some slight rearrangement of the constituent molecules1 since such stones are invariably characterized by high refraction and pronounced dichroism1 the crystalline symmetry1 however1 remaining unaltered1 the cause must be attributed to some change in the tinctorial agent1 probably oxidation4 ,the yellow stones from ,ceylon1 if treated in a similar manner1 lose their colour entirely4 ,the pale yellow-brown stones from ,russia fade on prolonged exposure to strong sunlight1 for which reason the superb suite of crystals from the ,urulga ,river1 which came with the ,koksharov collection to the ,british ,museum1 are kept under cover4 ,the name of the species is derived from topazion 71 to seek71 the name given to an island in the ,red ,sea1 which in olden times was with difficulty located1 but it was applied by ,pliny and his contemporaries to the yellowish peridot found there4 ,the term was applied in the ,middle ,ages loosely to any yellow stone1 and was gradually applied more particularly to the stone that was then more prevalent1 the topaz of modern science4 ,as has already been pointed out 7p4 #aaa71 the term is still employed in jewellery to signify any yellow stone4 ,the true topaz was probably included by ,pliny under the name chrysolithus4 ,illustration3 ,,fig4 #gb4--,topaz ,crystal4 ,the symmetry is orthorhombic1 and the crystals are prismatic in shape and terminated by numerous inclined faces1 and usually by a large face perpendicular to the prism edge 7,fig4 #gb74 ,topaz cleaves with great readiness at right angles to the prism edge2 owing to its facile cleavage1 flaws are easily started1 and caution must be exercised not to damage a stone by knocking it against hard and unyielding substances4 ,the dichroism of a yellow topaz is always perceptible1 one of the twin colours being distinctly more reddish than the other1 and the phenomenon is very marked in the case of stones the colour of which has been artificially altered to pink4 ,the values of the least and the greatest of the principal indices of refraction vary from #afae to #afbi1 and from #afbe to #afcg1 respectively1 the double refraction being about #jjaj in amount1 and positive in sign4 ,the high values correspond to the altered stones4 ,the specific gravity1 the mean value of which is #cee with a variation of #jje on either side1 is higher than would be expected from the refractivity4 ,a cleavage flake exhibits in convergent polarized light a wide-angled biaxial picture1 the #8eyes' lying outside the field of view4 ,the relation of the principal optical directions and the directions of single refraction to the crystal are shown in ,fig4 #bg4 ,the hardness is #h on ,mohs's scale1 and in this character it is surpassed only by chrysoberyl1 corundum1 and diamond4 ,topaz is pyro-electric1 in which respect tourmaline alone exceeds it1 and it may be strongly electrified by friction4 ,although the range of refraction overlaps that of tourmaline1 there is no risk of confusion1 because the latter has nearly thrice the amount of double refraction 7cf4 p4 #bi74 ,apart from the difference in refraction1 a yellow topaz ought never to be confused with a yellow quartz1 because the former sinks1 and the latter floats in methylene iodide4 ,the same test distinguishes topaz from beryl1 and1 indeed1 from tourmaline also4 ,judged by the criterion of price1 topaz is not in the first rank of precious stones4 ,stones of good colour and free from flaws are now1 however1 scarce4 ,pale stones are worth very little1 possibly less than #ds4 a carat1 but the price rapidly advances with increase in colour1 reaching #bjs4 for yellow1 #hjs4 for pink and blue stones4 ,since topazes are procurable in all sizes customary in jewellery1 the rates vary but slightly1 if at all1 with the size4 ,topaz occurs principally in pegmatite dykes and in cavities in granite1 and is interesting to petrologists as a conspicuous instance of the result of the action of hot acid vapours upon rocks rich in aluminium silicates4 ,magnificent crystals have come from the extensive mining district which stretches along the eastern flank of the ,ural ,mountains1 and from the important mining region surrounding ,nertschinsk1 in the ,government of ,transbaikal1 ,siberia4 ,fine green and blue stones have been found at ,alabashka1 near ,ekaterinburg1 in the ,government of ,perm1 and at ,miask in the ,ilmen ,mountains1 in the ,government of ,orenburg4 ,topazes of the rare reddish hue have been picked out from the gold washings of the ,sanarka ,river1 ,troitsk1 also in the ,government of ,orenburg4 ,splendid pale-brown stones have issued from the ,urulga ,river1 near ,nertschinsk1 and good crystals have come from the ,adun-,tschilon ,mountains4 ,kamchatka has produced yellow1 blue1 and green stones4 ,in the ,british ,isles1 beautiful sky-blue1 waterworn crystals have been found at ,cairngorm1 ,banffshire1 in ,scotland1 and colourless stones in the ,mourne ,mountains1 ,ireland1 and at ,st4 ,michael's ,mount1 ,cornwall4 ,most of the topazes used in jewellery of the present day come from either ,brazil or ,ceylon4 ,ouro ,preto1 ,villa ,rica1 and ,minas ,novas1 in the ,state of ,minas ,geraes1 are the principal localities in ,brazil4 ,numerous stones1 often waterworn1 brilliant and colourless or tinted lovely shades of blue and wine-yellow1 occur there2 reddish stones also have been found at ,ouro ,preto4 ,ceylon furnishes a profusion of yellow1 light-green1 and colourless1 waterworn pebbles4 ,the colourless stones found there are incorrectly termed by the natives #8water-sapphire1' and the light-green stones are sold with beryl as aquamarines2 the stones locally known as #8king topaz' are really yellow corundum 7cf4 p4 #aha74 ,colourless crystals1 sometimes with a faint tinge of colour1 have been discovered in many parts of the world1 such as ,ramona1 ,san ,diego ,county1 ,california1 and ,pike's ,peak1 ,colorado1 in the ,united ,states1 ,san ,luis ,potosi in ,mexico1 and ,omi and ,otami-yama in ,japan4 ,,chapter ,,xxii ,,spinel 7,balas-,ruby1 ,rubicelle7 ,spinel labours under the serious disadvantage of being overshadowed at almost all points by its opulent and more famous cousins1 sapphire and ruby1 and is not so well known as it deserves to be4 ,the only variety which is valued as a gem is the rose-tinted stone called balas-ruby 7,plate ,,xxvii1 ,fig4 #c71 which is very similar to the true ruby in appearance2 they are probably often confused1 especially since they are found in intimate association in nature4 ,spinels of other colours are not very attractive to the eye1 and are not likely to be in much demand4 ,blue spinel 7,plate ,,xxvii1 ,fig4 #d7 is far from common1 but the shade is inclined to steely-blue1 and is much inferior to the superb tint of the true sapphire4 ,spinel is very hard and eminently suitable for a ring-stone1 but is seldom large and transparent enough for larger articles of jewellery4 ,spinel is an aluminate of magnesium corresponding to the formula ,mg,al#b,o#d1 and therefore is closely akin to corundum1 alumina1 and chrysoberyl1 aluminate of beryllium4 ,the composition may1 however1 vary considerably owing to the isomorphous replacement of one element by another2 in particular1 ferrous oxide or manganese oxide often takes the place of some magnesia1 and ferric oxide or chromic oxide is found instead of part of the alumina4 ,when pure1 spinel is devoid of colour1 but such stones are exceedingly rare4 ,no doubt chromic oxide is responsible for the rose-red hue of balas-ruby1 and also1 when tempered by ferric oxide1 for the orange tint of rubicelle1 and manganese is probably the cause of the peculiar violet colour of almandine-spinel4 ,it is scarcely possible to define all the shades between blue and red that may be assumed by spinel4 ,stones which are rich in iron are known as pleonaste or ceylonite2 they are quite opaque1 but are sometimes used for ornamental wear4 ,the name of the species comes from a diminutive form of 1 a spark1 and refers to the fiery red colour of the most valued kind of spinel4 ,it may be noted that the ,latin equivalent of the word1 carbunculus1 has been applied to the crimson garnet when cut en cabochon4 ,balas is derived from ,balascia1 the old name for ,badakshan1 the district from which the finest stones were brought in medival times4 ,illustration3 ,,figs4 #gc1 #gd4--,spinel ,crystals4 ,spinel1 like diamond1 belongs to the cubic system of crystalline symmetry1 and occurs in beautiful octahedra1 or in flat triangular-shaped plates 7,figs4 #gc1 #gd7 the girdles of which are cleft at each corner1 these plates being really twinned octahedra4 ,the refraction is1 of course1 single1 and there is therefore no double refraction or dichroism2 this test furnishes the simplest way of discriminating between the balas and the true ruby4 ,owing to isomorphous replacement the value of the refractive index may lie anywhere between #agaf and #agcf4 ,the lower values1 about #agbj1 correspond to the most transparent red and blue stones2 the deep violet stones have values above #agcj4 ,spinel possesses little colour-dispersion1 or #8fire4' ,in the same way the values of the specific gravity1 even of the transparent stones1 vary between #ce and #cg1 but the opaque ceylonite has values as high as #da4 ,spinel is slightly softer than sapphire and ruby1 and has the symbol #h on ,mohs's scale1 and it is scarcely inferior in lustre to these stones4 ,spinel is easily separated from garnet of similar colour by its lower refractivity4 ,spinels run from #ajs4 to #e a carat1 depending on their colour and quality1 and exceptional stones command a higher rate4 ,spinel always occurs in close association with corundum4 ,the balas and the true ruby are mixed together in the limestones of ,burma and ,siam4 ,curiously enough1 the spinel despite its lower hardness is found in the river gravels in perfect crystals1 whereas the rubies are generally waterworn4 ,fine violet and blue spinels occur in the prolific gem-gravels of ,ceylon4 ,a large waterworn octahedron and a rough mass1 both of a fine red colour1 are exhibited in the ,mineral ,gallery of the ,british ,museum 7,natural ,history71 and a beautiful faceted blue stone is shown close by4 ,the enormous red stone1 oval in shape1 which is set in front of the ,english crown1 is not a ruby1 as it was formerly believed to be1 but a spinel4 ,it was given to the gallant ,black ,prince by ,pedro the ,cruel after the battle of ,najera in #acfg1 and was subsequently worn by ,henry ,v upon his helmet at the battle of ,agincourt4 ,as usual with ,indian-fashioned stones it is pierced through the middle1 but the hole is now hidden by a small stone of similar colour4 ,the ,british ,regalia also contains the famous stone called the ,timur ,ruby or ,khiraj-i-,alam 7,tribute of the ,world71 which weighs just over #ceb carats1 and is the largest spinel-ruby known4 ,it is uncut1 but polished4 ,its history goes back to #acih1 when it was captured by the ,amir ,timur at ,delhi4 ,on the wane of the ,tartar empire the stone became the property of the ,shahs of ,persia1 until it was given by ,abbas ,i to his friend and ally1 the ,mogul ,emperor1 ,jehangir4 ,it remained at ,delhi until1 on the sack of that city by ,nadir ,shah in #agci1 it1 together with immense booty1 including the ,koh-i-nor1 fell into the hands of the conqueror4 ,like the great diamond1 it eventually came into the possession of ,runjit ,singh at ,lahore1 and on the annexation of the ,punjab in #ahej passed to the ,east ,india ,company4 ,it was shown at the ,great ,exhibition of #ahea1 and afterwards presented to ,queen ,victoria4 ,mention has been made above 7p4 #aba7 of the blue spinel which is manufactured in imitation of the true sapphire4 ,the artificial stone is quite different in tint from the blue spinel found in nature4 ,,chapter ,,xxiii ,,garnet ,the important group of minerals which are known under the general name of garnet provides an apt illustration of the fact that rarity is an essential condition if a stone is to be accounted precious4 ,owing to the large quantity of ,bohemian garnets1 of a not very attractive shade of yellowish red1 that have been literally poured upon the market during the past half-century the species has become associated with cheap and often ineffective jewellery1 and has acquired a stigma which completely prevents its attaining any popularity with those professing a nice taste in gem-stones4 ,it must not1 however1 be supposed that garnet has entirely disappeared from high-class jewellery although the name may not readily be found in a jeweller's catalogue4 ,those whose business it is to sell gem-stones are fully alive to the importance of a name1 and1 as has already been remarked 7p4 #aji71 they have been fain to meet the prejudices of their customers by offering garnets under such misleading guises as #8,cape-ruby1' #8,uralian emerald1' or #8olivine4' ,garnets may1 moreover1 figure under another name quite unintentionally4 ,probably many a fine stone masquerades as a true ruby2 the impossibility of distinguishing these two species in certain cases by eye alone is perhaps not widely recognized4 ,an instructive instance came under the writer's notice a few years ago4 ,a lady one day had the misfortune to fracture one of the stones in a ruby ring that had been in the possession of her family for upwards of a century1 and was originally purchased of a leading firm of jewellers in ,london4 ,she took the ring to her jeweller1 and asked him to have the stone replaced by another ruby4 ,a day or two later he sent word that it was scarcely worth while to put a ruby in because the stones in the ring were paste4 ,naturally distressed at such an opinion of a ring which had always been held in great esteem by her family1 the lady consulted a friend1 who suggested showing it to the writer4 ,a glance was sufficient to prove that if the ring had been in use so long the stones could not possibly be paste on account of the excellent state of their polish1 but a test with the refractometer showed that the stones were really almandine-garnets1 which so often closely resemble the true ruby in appearance4 ,beautiful as the stones were1 the ring was probably not worth one-tenth what the value would have been had the stones been rubies4 ,to the student of mineralogy garnet is for many reasons of peculiar interest4 ,it affords an excellent illustration of the facility which certain elements possess for replacing one another without any great disturbance of the crystalline form4 ,despite their apparent complexity in composition all garnets conform to the same type of formula3 lime1 magnesia1 and ferrous and manganese oxides1 and again alumina and ferric and chromic oxides may replace each other in any proportion1 iron being present in two states of oxidation1 and it would be rare to find a stone which agrees in composition exactly with any of the different varieties of garnet given below4 ,illustration3 ,,figs4 #ge1 #gf4--;,garnet ,crystals4 ,garnet belongs to the cubic system of crystalline symmetry4 ,its crystals are commonly of two kinds1 both of which are very characteristic1 the regular dodecahedron1 i4e4 twelve-faced figure 7,fig4 #ge71 and the tetrakis-octahedron or three-faced octahedron 7,fig4 #gf72 the latter crystals are1 especially when weather- or water-worn1 almost spherical in shape4 ,closer and more refined observations have shown that garnet is seldom homogeneous1 being usually composed of several distinct individuals of a lower order of symmetry4 ,although singly refractive as far as can be determined with the refractometer or by deviation through a prism1 yet when examined under the polarizing microscope1 garnets display invariably a small amount of local double refraction4 ,the transition from light to darkness is1 however1 not sharp as in normal cases1 but is prolonged into a kind of twilight4 ,in hardness1 garnet is on the whole about the same as quartz1 but varies slightly2 hessonite and andradite are a little softer1 pyrope1 spessartite1 and almandine are a little harder1 while uvarovite is almost the same4 ,all the varieties except uvarovite are fusible when heated before the blowpipe1 and small fragments melt sufficiently on the surface in the ordinary bunsen flame to adhere to the platinum wire holding them4 ,this test is very useful for separating rough red garnets1 pyrope or almandine1 from red spinels or zircons of very similar appearance4 ,far greater variation occurs in the other physical characters4 ,the specific gravity may have any value between #cee and #dbj1 and the refractive index ranges between #agdj and #ahij4 ,both the specific gravity and the refractive index increase on the whole with the percentage amount of iron4 ,garnet is a prominent constituent of many kinds of rocks1 but the material most suitable for gem purposes occurs chiefly in crystalline schists or metamorphic limestones4 ,pyrope and demantoid are furnished by peridotites and the serpentines resulting from them2 almandine and spessartite come mostly from granites4 ,the name of the species is derived from the ,latin granatus1 seed-like1 and is suggested by the appearance of the spherical crystals when embedded in their pudding-like matrix4 ,the varieties most adapted to jewellery are the fiery-red pyrope and the crimson and columbine-red almandine2 the closer they approach the ruddy hue of ruby the better they are appreciated4 ,hessonite was at one time in some demand1 but it inclines too much to the yellowish shade of red and possesses too little perfection of transparency to accord with the taste of the present day4 ,demantoid provides beautiful1 pale and dark emerald-green stones1 of brilliant lustre and high dispersion1 which are admirably adapted for use in pendants or necklaces2 on account of their comparative softness it would be unwise to risk them in rings4 ,in many stones the colour takes a yellowish shade1 which is less in demand4 ,uvarovite also occurs in attractive emerald-green stones1 but unfortunately none as yet have been found large enough for cutting4 ,a few truly magnificent spessartites are known--one1 a splendid example1 weighing #f carats1 being in the possession of ,sir ,arthur ,church2 but the species is far too seldom transparent to come into general use4 ,the price varies per carat from #bs4 for common garnet to #ajs4 for stones most akin to ruby in colour1 and exceptional demantoids may realize even as much as #aj a carat4 ,the old style of cutting was almost invariably rounded or en cabochon1 but at the present day the brilliant-cut front and the step-cut back is most commonly adopted4 ,the several varieties will now be considered in detail4 7a7 ,,hessonite 7,grossular1 ,cinnamon-,stone1 ,hyacinth1 ,jacinth7 ,this variety1 strictly a calcium-aluminium garnet corresponding to the formula ,ca#c;,al#b7,si,o#d7c1 but generally containing some ferric oxide and therefore tending towards andradite1 is called by several different names4 ,in science it is usually termed grossular1 a word derived from grossularia1 the botanical name for gooseberry1 in allusion to the colour and appearance of many crystals1 or hessonite1 and less correctly essonite1 words derived from the ,greek in reference to the inferior hardness of these stones as compared with zircon of similar colour2 in jewellery it is better known as cinnamon-stone1 if a golden-yellow in colour1 or hyacinth or jacinth4 ,the last word1 which is indiscriminately used for hessonite and yellow zircon1 but should more properly be applied to the latter1 is derived from an old ,indian word 7cf4 p4 #bbi72 jewellers1 however1 retain it for the garnet4 ,only the yellow and orange shades of hessonite 7,plate ,,xxix1 ,fig4 #e7 are used for jewellery4 ,neither the brownish-green kind1 to which the term grossular may properly be applied1 nor the rose-red is transparent enough to serve as a gem-stone4 ,hessonite may mostly be recognized1 even when cut1 by the curiously granular nature of its structure1 just as if it were composed of tiny grains imperfectly fused together2 this appearance1 which is very characteristic1 may readily be perceived if the interior of the stone be viewed through a lens of moderate power4 ,the specific gravity varies from #cee to #cff1 and the refractive index from #agdb to #agdh4 ,the hardness is on the whole slightly below that of quartz4 ,when heated before a blowpipe it easily fuses to a greenish glass4 ,the most suitable material is found in some profusion in the gem-gravels of ,ceylon1 in which it is mixed up with zircon of an almost identical appearance2 both are called hyacinth4 ,hessonites from other localities1 although attractive as museum specimens1 are not large and clear enough for cutting purposes4 ,switzerland at one time supplied good stones1 but the supply has long been exhausted4 7b7 ,,pyrope 7#8,cape-,ruby'7 ,often quite ruby-red in colour 7,plate ,,xxix1 ,fig4 #f71 this variety is probably the most popular of the garnets4 ,it is strictly a magnesium-aluminium garnet corresponding to the formula ,mg#c;,al#b7,si,o#d7c1 but usually contains some ferrous oxide and thus approaches almandine4 ,both are included among the precious garnets4 ,its name is derived from 1 fire-like1 in obvious allusion to its characteristic colour4 ,although at its best pyrope closely resembles ruby1 its appearance is often marred by a tinge of yellow which decidedly detracts from its value4 ,pyrope generally passes as a variety of ruby1 and under such names as #8,cape-ruby1' #8,arizona-ruby1' depending on the origin of the stones1 commands a brisk sale4 ,the specific gravity varies upwards from #cgj1 depending upon the percentage amount of iron present1 and similarly the refractive index varies upwards from #agdj2 in the higher values pyrope merges into almandine4 ,its hardness is slightly greater than that of quartz1 and may be expressed on ,mohs's scale by the symbol #g4 ,an enormous quantity of small red stones1 mostly with a slight tinge of yellow1 have been brought to light at ,teplitz1 ,aussig1 and other spots in the ,bohemian ,mittelgebirge1 and a considerable industry in cutting and marting them has grown up at ,bilin4 ,fine ruby-red stones accompany diamond in the #8blue ground' of the mines at ,kimberley and also at the ,premier mine in the ,transvaal4 ,similar stones are also found in ,arizona and ,colorado in the ,united ,states1 and in ,australia1 ,rhodesia1 and elsewhere4 ,although commonly quite small in size1 pyrope has occasionally attained to considerable size4 ,according to ,de ,boodt the ,kaiser ,rudolph ,,ii had one in his possession valued at #de1jjj thalers 7about #fgej74 ,the ,imperial ,treasury at ,vienna contains a stone as large as a hen's egg4 ,another about the size of a pigeon's egg is in the famous ,green ,vaults at ,dresden1 and the ,king of ,saxony has one1 weighing #dfh carats1 set in an ,order of the ,golden ,fleece4 7c7 ,,rhodolite ,this charming pale-violet variety was found at ,cowee ,creek and at ,mason's ,branch1 ,macon ,county1 ,north ,carolina1 ,u4,s4,a41 but in too limited amount to assume the position in jewellery it might otherwise have expected4 ,in composition it lies between pyrope and almandine1 and may be supposed to contain a proportion of two molecules of the former to one of the latter4 ,its specific gravity is #chd1 refractive index #agfj1 and hardness #g4 ,it exhibits in the spectroscope the absorption-bands characteristic of almandine4 7d7 ,,almandine 7,carbuncle7 ,this variety is iron-aluminium garnet corresponding to the formula ,fe#c;,al#b7,si,o#d7c1 but the composition is very variable4 ,in colour it is deep crimson and violet or columbine-red 7,plate ,,xxix1 ,fig4 #h71 but with increasing percentage amount of ferric oxide it becomes brown and black1 and opaque1 and quite unsuitable for jewellery4 ,the name of the variety is a corruption of ,alabanda in ,asia ,minor1 where in ,pliny's time the best red stones were cut4 ,almandine is sometimes known as ,syriam1 or incorrectly ,syrian garnet1 because at ,syriam1 once the capital of the ancient kingdom of ,pegu1 which now forms part of ,lower ,burma1 such stones were cut and sold4 ,crimson stones1 cut in the familiar en cabochon form and known as carbuncles1 were extensively employed for enriching metalwork1 and a half-century or so ago were very popular for ornamental wear1 but their day has long since gone4 ,such glowing stones are aptly described by their name1 which is derived from the ,latin carbunculus1 a little spark4 ,in ,pliny's time1 however1 the term was used indiscriminately for all red stones4 ,it has already been remarked that the word spinel has a similar significance4 ,the specific gravity varies from #cij for transparent stones to #dbj for the densest black stones1 and the refractive index may be as high as #ahaj4 ,almandine is one of the hardest of the garnets1 and is represented by the symbol #g on ,mohs's scale4 ,the most interesting and curious feature of almandine lies in the remarkable and characteristic absorption-spectrum revealed when the transmitted light is examined with a spectroscope 7p4 #fa74 ,the phenomenon is displayed most vividly by the violet stones1 and is1 indeed1 the cause of their peculiar colour4 ,although a common mineral1 almandine of a quality fitted for jewellery occurs in comparatively few localities4 ,it is found in ,ceylon1 but not so plentifully as hessonite4 ,good stones are mined in various parts of ,india1 and are nearly all cut at ,delhi or ,jaipur4 ,brazil supplies good material1 especially in the ,minas ,novas district of ,minas ,geraes1 where it accompanies topaz1 and ,uruguay also furnishes serviceable stones4 ,almandine is found in ,australia1 and in many parts of the ,united ,states4 ,recently small stones of good colour have been discovered at ,luisenfelde in ,german ,east ,africa4 7e7 ,,spessartite ,properly a manganese-aluminium garnet corresponding to the formula ,mn#c;,al#b7,si,o#d7c1 this variety generally contains iron in both states of oxidation4 ,if only transparent and large enough its aurora-red colour would render it most acceptable in jewellery4 ,two splendid stones have1 indeed1 been found in ,ceylon 7p4 #baa71 and good stones rather resembling hessonites have been quarried at ,amelia ,court ,house in ,virginia1 and others have come from ,nevada2 otherwise1 spessartite is unknown as a gem-stone4 ,the specific gravity ranges from #dj to #dc1 and the refractive index is about #aha1 both characters being high2 the hardness is slightly greater than that of quartz4 7f7 ,,andradite 7,demantoid1 ,topazolite1 #8,olivine'7 ,andradite is strictly a calcium-iron garnet corresponding to the formula ,ca#c;,fe#b7,si,o#d7c1 but as usual the composition varies considerably4 ,it is named after d',andrada1 a ,portuguese mineralogist1 who made a study of garnet more than a century ago4 ,once contemptuously styled common garnet1 andradite suddenly sprang into the rank of precious stones upon the discovery some thirty years ago of the brilliant1 green stones 7,plate ,,xxix1 ,fig4 #g7 in the serpentinous rock beside the ,bobrovka stream1 a tributary of the ,tschussowaja ,river1 in the ,sissersk district on the western side of the ,ural ,mountains4 ,the shade of green varies from olive through pistachio to a pale emerald1 and is probably due to chromic oxide4 ,its brilliant lustre1 almost challenging that of diamond1 and its enormous colour-dispersion1 in which respect it actually transcends diamond1 raise it to a unique position among coloured stones4 ,unfortunately its comparative softness limits it to such articles of jewellery as pendants and necklaces1 where it is not likely to be rubbed4 ,when first found it was supposed to be true emerald1 which does actually occur near ,ekaterinburg1 and was termed #8,uralian emerald4' ,when analysis revealed its true nature1 it received from science the slightly inharmonious name of demantoid in compliment to its adamantine lustre4 ,jewellers1 however1 prefer to designate it #8olivine1' not very happily1 because the stones usually cut are not olive-green and the name is already in extensive use in science for a totally distinct species 7p4 #bbe72 they recognized the hopelessness of endeavouring to find a market for them as garnets4 ,the yellow kind of andradite known as topazolite would be an excellent gem-stone if only it were found large and transparent enough4 ,ordinary andradite is brown or black1 and opaque2 it has occasionally been used for mourning jewellery4 ,the specific gravity varies from #ch to #ci1 being about #che for demantoid1 which has a high refractive index1 varying from #ahhj to #ahij1 and may with advantage be cut in the brilliant form4 ,it is the softest of the garnets1 being only #f on ,mohs's scale4 7g7 ,,uvarovite ,this variety1 which is altogether unknown in jewellery1 is a calcium-iron garnet corresponding mainly to the formula ,ca#c;,cr#b7,si,o#d7c1 but with some alumina always present1 and was named after a ,russian minister4 ,it has an attractive green colour1 and is1 moreover1 hard1 being about on ,mohs's scale1 but it has never yet come to light of a size suitable for cutting4 ,the specific gravity is low1 varying from #cda to #ceb4 ,unlike the kindred varieties it cannot be fused by heating before an ordinary blowpipe4 ,,chapter ,,xxiv ,,tourmaline 7,rubellite7 ,tourmaline is unsurpassed even by corundum in variety of hue1 and it has during recent years rapidly advanced in public favour1 mainly owing to the prodigal profusion in which nature has formed it in that favoured ,state1 ,california1 the garden of the west4 ,its comparative softness militates against its use in rings1 but its gorgeous coloration renders it admirably fitted for service in any article of jewellery1 such as a brooch or a pendant1 in which a large central stone is required4 ,like all coloured stones it is generally brilliant-cut in front and step-cut at the back1 but occasionally it is sufficiently fibrous in structure to display1 when cut en cabochon1 pronounced chatoyancy4 ,the composition of this complex species has long been a vexed question among mineralogists1 but considerable light was recently thrown on the subject by ,schaller1 who showed that all varieties of tourmaline may be referred to a formula of the type #ab,si,o#b4c;,b#b,o#c47i-x77,al1,fe7#b,o #c4cx7,fe1,mn1,ca1 ,mg1,k#b1,na#b1,li#b1;,h#b7,o4#c;,h#b,o4 ,the ratios of boric oxide1 silica1 and water are nearly constant in all analyses1 but great variation is possible in the proportions of the other constituents4 ,having regard to this complexity1 it is not surprising to find that the range in colour is so great4 ,colourless stones1 to which the name achroite is sometimes given1 were at one time exceedingly rare1 but they are now found in greater number in ,california4 ,stones which are most suited to jewellery purposes are comparatively free from iron1 and apparently owe their wonderful tints to the alkaline earths2 lithia1 for instance1 is responsible for the beautiful tint of the highly prized rubellite1 and magnesia1 no doubt1 for the colour of the brown stones of various tints4 ,tourmaline rich in iron is black and almost opaque4 ,it is a striking peculiarity of the species that the crystals are rarely uniform in colour throughout1 the boundaries between the differently coloured portions being sharp and abrupt1 and the tints remarkably in contrast4 ,sometimes the sections are separated by planes at right angles to the length of the crystal1 and sometimes they are zonal1 bounded by cylindrical surfaces running parallel to the same length4 ,in the latter case a section perpendicular to the length shows zones of at least three contrasting tints4 ,in the ,brazilian stones the core is generally red1 bounded by white1 with green on the exterior1 while the reverse is the case in the ,californian stones1 the core being green or yellow1 bounded by white1 with red on the exterior4 ,tourmaline may1 indeed1 be found of almost every imaginable tint1 except1 perhaps1 the emerald green and the royal sapphire-blue4 ,the principal varieties are rose-red and pink 7rubellite7 7,plate ,,xxvii1 ,fig4 #a71 green 7,brazilian emerald71 indigo-blue 7indicolite71 blue 7,brazilian sapphire71 yellowish green 7,brazilian peridot7 7,plate ,,xxvii1 ,fig4 #b71 honey-yellow 7,ceylonese peridot71 violet-red 7siberite71 and brown 7,plate ,,xxvii1 ,fig4 #h74 ,the black1 opaque stones are termed schorl4 ,the name of the species is derived from the ,ceylonese word1 turamali1 and was first employed when a parcel of gem-stones was brought to ,amsterdam from ,ceylon in #agjc2 in ,ceylon1 however1 the term is applied by native jewellers to the yellow zircon commonly found in the island4 ,schorl1 the derivation of which is unknown1 is the ancient name for the species1 and is still used in that sense by miners1 but it has been restricted by science to the black variety4 ,the #8,brazilian emerald' was introduced into ,europe in the seventeenth century and was not favourably received1 possibly because the stones were too dark in colour and were not properly cut2 that they should have been confused with the true emerald is eloquent testimony to the extreme ignorance of the characters of gem-stones prevalent in those dark ages4 ,achroite comes from the ,greek1 1 without colour4 ,illustration3 ,,fig4 #gg4--,tourmaline ,crystal4 ,to the crystallographer tourmaline is one of the most interesting of minerals4 ,if the crystals1 which are usually prismatic in form1 are doubly terminated1 the development is so obviously different at the two ends 7,fig4 #gg7 as to indicate that directional character in the molecular arrangement1 termed the polarity1 which is borne out by other physical properties4 ,tourmaline is remarkably dichroic1 ,a brown stone1 except in very thin sections1 is practically opaque to the ordinary ray1 and consequently a section cut parallel to the crystallographic axis1 i4e4 to the length of a crystal prismatically developed1 transmits only the extraordinary ray4 ,such sections were in use for yielding plane-polarized light before ,nicol devised the calcite prism known by his name 7cf4 p4 #dd74 ,it is evident that tourmaline1 unless very light in tint1 must be cut with the table facet parallel to that axis1 because otherwise the stone will appear dark and lifeless4 ,the values of the extraordinary and ordinary refractive indices range between #afad and #afch1 and #afcc and #affi respectively2 the double refraction1 therefore1 is fairly large1 amounting to #jjbe1 and1 since the ordinary exceeds the extraordinary ray1 its character is negative4 ,the specific gravity varies from #cj to #cb4 ,the lower values in both characters correspond to the lighter coloured stones used in jewellery2 the black stones1 as might be expected from their relative richness in iron1 are the densest4 ,the hardness is only about the same as that of quartz1 or perhaps a little greater1 varying from #g to #g4 ,it will be noticed that the range of refractivity overlaps that of topaz 7q4v47 but the latter has a much smaller double refraction1 and may thus be distinguished 7p4 #bi74 ,unmounted stones are still more easily distinguished1 because tourmaline floats in methylene iodide1 while topaz sinks4 ,the pyro-electric phenomenon 7cf4 p4 #hb7 for which tourmaline is remarkable1 although of little value as a test in the case of a cut stone1 is of great scientific interest1 because it is strong evidence of the peculiar crystalline symmetry pertaining to its molecular arrangement4 ,tourmalines range in price from #es4 to #bjs4 a carat according to their colour and quality1 but exceptional stones may command a higher rate4 ,tourmaline is usually found in the pegmatite dykes of granites1 but it also occurs in schists and in crystalline limestones4 ,rubellite is generally associated with the lithia mica1 lepidolite2 the groups of delicate pink rubellite bespangling a background of greyish white lepidolite are among the most beautiful of museum specimens4 ,magnificent crystals of pink1 blue1 and green tourmaline have been found in the neighbourhood of ,ekaterinburg1 principally at ,mursinka1 in the ,urals1 ,russia1 and fine rubellite has come from the ,urulga ,river1 and other spots near ,nertschinsk1 ,transbaikal1 ,asiatic ,russia4 ,elba produces pink1 yellowish1 and green stones1 frequently particoloured2 sometimes the crystals are blackened at the top1 and are then known locally as #8nigger-heads4' ,ceylon supplies small yellow stones--the original tourmaline--which are confused with the zircon of a similar colour1 and rubellite accompanies the ruby at ,ava1 ,burma4 ,beautiful crystals1 green and red1 often diversely coloured1 come from various parts1 such as ,minas ,novas and ,arassuahy1 of the ,state of ,minas ,geraes1 ,brazil4 ,suitable gem material has been found in numerous parts of the ,united ,states4 ,paris and ,hebron in ,maine have produced gorgeous pink and green crystals1 and ,auburn in the same state has supplied deep-blue1 green1 and lilac stones4 ,fine crystals1 mostly green1 but also pink and particoloured1 occur in an albite quarry near the ,conn ,river at ,haddam ,neck1 ,connecticut4 ,all former localities have1 however1 been surpassed by the extraordinary abundance of superb green1 and especially pink1 crystals at ,pala and ,mesa ,grande in ,san ,diego ,county1 ,california4 ,as elsewhere1 many-hued stones are common4 ,the latter locality supplies the more perfectly transparent crystals4 ,kunz states that two remarkable rubellite crystals were found there1 one being #de mm4 in length and #db mm4 in diameter1 and the other #ef mm4 in length and #bd mm4 in diameter4 ,madagascar1 which has proved of recent years to be rich in gem-stones1 supplies green1 yellow1 and red stones1 both uniformly tinted and particoloured1 which in beauty1 though perhaps not in size1 bear comparison with any found elsewhere4 ,,chapter ,,xxv ,,peridot ,the beautiful bottle-green stone1 which from its delicate tint has earned from appreciative admirers the poetical sobriquet of the evening emerald1 and which has during recent years crept into popular favour and now graces much of the more artistic jewellery1 is named as a gem-stone peridot--a word long in use among ,french jewellers1 the origin and meaning of which has been forgotten--but is known to science either as olivine1 on account of the olive-green colour sometimes characterizing it1 or as chrysolite4 ,it is of interest to note that the last word1 derived from 1 golden1 and 1 stone1 was in use at the time of ,pliny1 but was employed for topaz and other yellow stones1 while his topaz1 curiously enough1 designated the modern peridot 7cf4 p4 #aii71 an inversion that has occurred in other words4 ,the true olivine must not be confused with the jewellers' #8olivine1' which is a green garnet from the ,ural ,mountains 7p4 #bag74 ,peridot is comparatively soft1 the hardness varying from #f to #g on ,mohs's scale1 and is suitable only for articles which are not likely to be scratched2 the polish of a peridot worn in a ring would soon deteriorate4 ,the choicest stones are in colour a lovely bottle-green 7,plate ,,xxix1 ,fig4 #b7 of various depths2 the olive-green stones 7,plate ,,xxix1 ,fig4 #c7 cannot compare with their sisters in attractiveness4 ,the step form of cutting is considered the best for peridot1 but it is sometimes cut round or oval in shape1 with brilliant-cut fronts4 ,peridot is a silicate of magnesium and iron1 corresponding to the formula 7,mg1,fe7#b,si,o#d1 ferrous iron1 therefore1 replacing magnesia4 ,to the ferrous iron it is indebted for its colour1 the pure magnesium silicate being almost colourless1 and the olive tint arises from the oxidation of the iron4 ,the latitude in the composition resulting from this replacement is evinced in the considerable range that has been observed in the physical characters1 but the crystalline symmetry persists unaltered2 the lower values correspond to the stones that are usually met with as gems4 ,peridot belongs to the orthorhombic system of crystalline symmetry1 and the crystals1 which display a large number of faces1 are prismatic in form and generally somewhat flattened4 ,the stones1 however1 that come into the market for cutting as gems are rarely unbroken4 ,the dichroism is rather faint1 one of the twin colours being slightly more yellowish than the other1 but it is more pronounced in the olive-tinted stones4 ,the values of the least and greatest of the principal indices of refraction vary greatly1 from #afej and #afhc to #affh and #agja1 but the double refraction1 amounting to #jjcc1 remains unaffected4 ,peridot1 though surpassed by sphene in extent of double refraction1 easily excels all the ordinary gem-stones in this respect1 and this character is readily recognizable in a cut stone by the apparent doubling of the opposite edges when viewed through the table facet 7cf4 p4 #da74 ,an equally large variation occurs in the specific gravity1 namely1 from #cc to #ce4 ,illustration3 ,,plate ,,xxvii #a4 ,,rubellite #b4 ,,tourmaline #c4 ,,balas-,,ruby #d4 ,,blue ,,spinel #e4 ,,quartz #f4 ,,white ,,opal #g4 ,,amethyst #h4 ,,tourmaline #i4 ,,black ,,opal #aj4 ,,fire ,,opal #aa4 ,,alexandrite 7,by daylight7 #ab4 ,,chrysoberyl #ac4 ,,alexandrite 7,by artificial light7 ,,gem-,,stones ,peridots of deep bottle-green hue command moderate prices at the present day1 about #cjs4 a carat being asked for large stones2 the paler tinted stones run down to a few shillings a carat4 ,the rate per carat may be very much larger for stones of exceptional size and quality4 ,olivine1 to use the ordinary mineralogical term1 is a common and important constituent of certain kinds of igneous rocks1 and it is also found in those strange bodies1 meteorites1 which come to us from outer cosmical space4 ,except in basaltic lavas1 it occurs in grains and rarely in well-shaped crystals4 ,stones that are large and transparent enough for cutting purposes come almost entirely from the island ,zebirget or ,st4 ,john situated on the west coast of the ,red ,sea1 opposite to the port of ,berenice4 ,this island belongs to the ,khedive of ,egypt1 and is at present leased to a ,french syndicate4 ,it is believed to be the same as the mysterious island which produced the #8topaz' of ,pliny's time4 ,magnificent stones have been discovered here1 rich green in colour1 and #bj to #cj1 and occasionally as much as #hj1 carats in weight when cut2 a rough mass attained to the large weight of #aij carats4 ,pretty1 light-green stones are supplied by ,queensland1 and peridots of a less pleasing dark-yellowish shade of green1 and without any sign of crystal form1 have during recent years come from ,north ,america4 ,stones rather similar to those from ,queensland have latterly been found in the ,bernardino ,valley in ,upper ,burma1 not far from the ruby mines4 ,,chapter ,,xxvi ,,zircon 7,jargoon1 ,hyacinth1 ,jacinth7 ,zircon1 which1 if known at all in jewellery1 is called by its variety names1 jargoon and hyacinth or jacinth1 is a species that deserves greater recognition than it receives4 ,the colourless stones rival even diamond in splendour of brilliance and display of #8fire'2 the leaf-green stones 7,plate ,,xxix1 ,fig4 #ac7 possess a restful beauty that commends itself2 the deep-red stones 7,plate ,,xxix1 ,fig4 #ad71 if somewhat sombre1 have a certain grandeur2 and no other species produces such magnificent stones of golden-yellow hue 7,plate ,,xxix1 ,fig4 #ab74 ,zircon is well known in ,ceylon1 which supplies the world with the finest specimens1 and is highly appreciated by the inhabitants of that sunny isle1 but it scarcely finds a place in jewellery elsewhere4 ,the colourless stones are cut as brilliants1 but brilliant-cut fronts with step-cut backs is the usual style adopted for the coloured stones4 ,zircon is a silicate of zirconium corresponding to the formula ,zr,si,o#d1 but uranium and the rare earths are generally present in small quantities4 ,the aurora-red variety is known as hyacinth or jacinth1 and the term jargoon is applied to the other transparent varieties1 and especially to the yellow stones4 ,the most attractive colours shown by zircon are leaf-green1 golden-yellow1 and deep red4 ,other common colours are brown1 greenish1 and sky-blue4 ,colourless stones are not found in nature1 but result from the application of heat to the yellow and brown stones4 ,the name of the species is ancient1 and comes from the ,arabic zarqn1 vermilion1 or the ,persian zarqn1 gold-coloured4 ,from the same source in all probability is derived the word jargoon through the ,french jargon and the ,italian giacone4 ,hyacinth 7cf4 p4 #baa7 is transliterated from the ,greek 1 itself adapted from an old ,indian word2 it is in no way connected with the flower of the same name4 ,the last word has seen some changes of meaning4 ,in ,pliny's time yellow zircons were indiscriminately classified with other yellow stones as chrysolite4 ,his hyacinth was used for the sapphire of the present day1 but was subsequently applied to any transparent corundum4 ,upon the introduction of the terms1 sapphire and ruby1 for the blue and the red corundum hyacinth became restricted to the other varieties1 of which the yellow was the commonest4 ,in the darkness of the ,middle ,ages it was loosely employed for all yellow stones emanating from ,india1 and was finally1 with increasing discernment in the characters of gem-stones1 assigned to the yellow zircon1 since it was the commonest yellow stone from ,india4 ,illustration3 ,,fig4 #gh4--,zircon ,crystal4 ,considered from the scientific point of view1 zircon is by far the most interesting and the most remarkable of the gem-stones4 ,the problem presented by its characters and constitution is one that still awaits a satisfactory solution4 ,certain zircons1 which are found as rolled pebbles in ,ceylon and never show any trace of crystalline faces1 have very nearly single refraction1 and the values of the refractive index vary from #agij to #ahdj1 and the specific gravity is about #djj to #dad1 and the hardness is slightly greater than that of quartz1 being about #g4 ,on the other hand1 such stones as the red zircons from ,expailly have remarkably different properties4 ,they show crystalline faces with tetragonal symmetry1 the faces present being four prismatic faces mutually intersecting at right angles and four inclined faces at each end 7,fig4 #gh74 ,they have large double refraction1 varying from #jjdd to #jjfb1 which is readily discerned in a cut stone 7cf4 p4 #da71 and the refractive indices are high1 the ordinary index varying from #aibc to #aica and the extraordinary from #aifg to #aiic4 ,since the ordinary is less than the extraordinary index the sign of the double refraction is positive4 ,the specific gravity likewise is much higher1 varying from #dfg to #dga4 ,the second type1 therefore1 sinks in molten silver-thallium nitrate1 whereas the first type floats4 ,the second type is also slightly harder1 being about #g on ,mohs's scale4 ,by heating either of these types the physical characters are not much altered1 except that the colour is weakened or entirely driven off and some change takes place in the double refraction4 ,but between these two types may be found zircons upon which the effect of heating is striking4 ,they seem to contract in size so that the specific gravity increases as much as three units in the first place of decimals1 and a corresponding increase takes place in the refractive indices1 and in the amount of double refraction4 ,the cause of these changes remains a matter of speculation4 ,evidently a third type of zircon exists which is capable of most intimate association with either of the other types1 and which is very susceptible to the effect of heat4 ,it may be noted that stones of the intermediate type are usually characterized by a banded or zonal structure suggesting a want of homogeneity4 ,the theory has been advanced that zircon contains an unknown element which has not yet been separated from zirconium4 ,zircon of the first type favours green1 sky-blue1 and golden-yellow colours2 honey-yellow1 light green1 blue1 and red colours characterize the second type2 and the intermediate stones are mostly yellowish green1 cloudy blue1 and green4 ,it is another peculiarity of zircon that it sometimes shows in the spectroscope absorption bands 7p4 #fa71 which were observed in #ahff by ,church4 ,many zircons do not exhibit the bands at all1 and others only display the two prominent bands in the red end of the spectrum4 ,of all the gem-stones zircon alone approaches diamond in brilliance of lustre1 and it also possesses considerable #8fire'2 it can1 of course1 be readily distinguished by its inferior hardness1 but a judgment based merely on inspection by eye might easily be erroneous4 ,according to ,church1 who has made a lifelong study of zircon1 the green and yellowish stones of the first variety emit a brilliant orange light when being ground on a copper wheel charged with diamond dust1 and the golden stones of the intermediate type glow with a fine orange incandescence in the flame of a bunsen burner2 the latter phenomenon is supposed to be due to the presence of thoria4 ,the leaf-green stones almost invariably show a series of parallel bands in the interior4 ,zircons vary from #es4 to #aes4 a carat1 but exceptional stones may be worth more4 ,by far the finest stones come from ,ceylon4 ,the colourless stones are there known as #8,matura diamonds1' and the hyacinth includes garnet 7hessonite7 of similar colour1 which is found with it in the same gravels4 ,the stones are always water-worn4 ,small hyacinths and deep-red stones come from ,expailly1 ,auvergne1 ,france1 and yellowish-red crystals are found in the ,ilmen ,mountains1 ,orenburg1 ,russia4 ,remarkably fine red stones have been discovered at ,mudgee1 ,new ,south ,wales1 and yellowish-brown stones accompany diamond at the ,kimberley mines1 ,south ,africa4 ,,chapter ,,xxvii ,,chrysoberyl 7,chrysolite1 ,cat's-,eye1 ,cymophane1 ,alexandrite7 ,chrysoberyl has at times enjoyed fleeting popularity on account of the excellent cat's-eyes cut from the fibrous stones1 and in the form of alexandrite it meets with a steadier1 if still limited1 demand4 ,it is a gem-stone that is seldom met with in ordinary jewellery1 although its considerable hardness befits it for all such purposes4 ,chrysoberyl is in composition an aluminate of beryllium corresponding to the formula ,be,al#b,o#d1 and is therefore closely akin to spinel4 ,it usually contains some ferric and chromic oxides in place of alumina1 and ferrous oxide in place of beryllia1 and it is to these accessory constituents that its tints are due4 ,other gem-stones containing the uncommon element beryllium are phenakite and beryl4 ,pale yellowish green1 the commonest colour1 is supposed to be caused by ferrous oxide2 such stones are known to jewellers as chrysolite 7,plate ,,xxvii1 ,fig4 #ab74 ,cat's-eyes 7,plate ,,xxix1 ,fig4 #a7 have often also a brownish shade of green4 ,the bluish green and dark olive-green stones known as alexandrite 7,plate ,,xxvii1 ,figs4 #aa1 #ac7 differ in appearance so markedly from their fairer sisters that their common parentage seems almost incredible4 ,the dull fires that glow within them1 and the curious change that comes over them at night1 add a touch of mystery to these dark stones4 ,chromic oxide is held responsible for their colour4 ,the cat's-eyes are1 of course1 always cut en cabochon1 but otherwise chrysoberyl is faceted4 ,the name of the species is composed of two ,greek words1 1 golden1 and 1 beryl1 and etymologically more correctly defines the lighter-coloured stones1 which were1 indeed1 at one time the only kind known4 ,chrysolite from 1 golden1 and 1 stone1 has much the same significance4 ,this name is preferred by jewellers1 but in science it is applied to an entirely different species1 which is known in jewellery as peridot4 ,cymophane1 from 1 wave1 and 1 appear1 refers to the peculiar opalescence characteristic of cat's-eyes2 it is sometimes used to designate these stones1 but does not find a place within the vocabulary of jewellery4 ,alexandrite is named after ,alexander ,,ii1 ,czar of ,russia1 because it first came to light on his birthday4 ,that circumstance1 coupled with its display of the national colours1 green and red1 and its at one time restriction to the mining district near ,ekaterinburg1 renders it dear to the heart of all loyal ,russians4 ,chrysoberyl crystallizes in the orthorhombic system1 and occurs in rather dull1 complex crystals1 which are sometimes so remarkably twinned1 especially in the variety called alexandrite1 as to simulate hexagonal crystals4 ,in keeping with the crystalline symmetry it is doubly refractive and biaxial1 having two directions of single refraction4 ,the least and the greatest of the principal indices of refraction may have any values between #agdb and #agdi1 and #agej and #ageg1 respectively1 the maximum amount of double refraction remaining always the same1 namely1 #jjji4 ,the mean principal refractive index is close to the least2 the sign of the double refraction is therefore positive1 and the shadow-edge corresponding to the lower index1 as seen in the refractometer1 has little1 if any1 perceptible motion when the stone is rotated4 ,the converse is the case with corundum2 the sign is negative1 and it is the shadow-edge corresponding to the greater refractive index that remains unaltered in position on rotation of the stone4 ,this test would suffice to separate chrysoberyl from yellow corundum1 even if the refractive indices of the former were not sensibly lower than those of the latter4 ,also1 the dichroism of chrysolite is stronger than that of yellow sapphires4 ,in alexandrite this phenomenon is most prominent2 the absorptive tints1 columbine-red1 orange1 and emerald-green1 corresponding to the three principal optical directions1 are in striking contrast1 and the first differs so much from the intrinsic colour of the stone as to be obvious to the unaided eye1 and is the cause of the red tints visible in a cut stone4 ,the curious change in colour of alexandrite1 from leaf-green to raspberry-red1 that takes place when the stone is seen by artificial light1 is due to a different cause1 as has been pointed out above 7p4 #ed74 ,the effect is illustrated by ,figs4 #aa1 #ac on ,plate ,,xxvii1 which represent a fine ,ceylon stone as seen by daylight and artificial light2 the influence of dichroism may be noticed in the former picture4 ,the specific gravity of chrysoberyl varies from #cfh to #cgh4 ,in hardness this species ranks above spinel and comes next to corundum1 being given the symbol #h on ,mohs's scale4 ,certain stones contain a multitude of microscopic channels arranged in parallel position4 ,when the stones are cut with their rounded surface parallel to the channels1 a broadish band of light is visible running across the stone at right angles to them1 and suggests the pupil of a cat's eye1 whence the common name for the stones4 ,the fact that the channels are hollow causes an opalescence1 which is absent from the quartz cat's-eye4 ,the most important locality for the yellowish chrysoberyl is the rich district of ,minas ,novas1 ,minas ,geraes1 ,brazil1 where it occurs in the form of pebbles1 and excellent material is also supplied by ,ceylon1 in both crystals and rounded pebbles4 ,other places for chrysolite are ,haddam1 ,connecticut1 and ,greenfield1 ,saratoga ,county1 ,new ,york1 in the ,united ,states1 and recently in the gem-gravels near the ,somabula ,forest1 ,rhodesia4 ,ceylon supplies some of the best cat's-eyes4 ,alexandrite was first discovered1 as already stated1 at the emerald mines near ,ekaterinburg1 in the ,urals2 but the supply is now nearly exhausted4 ,a poorer quality comes from ,takowaja1 also in the ,urals4 ,good alexandrite has come to light in ,ceylon1 and most of the stones that are placed on the market at the present day have emanated from that island4 ,the ,ceylon stones reach a considerable size1 often as much as from #aj to #bj carats in weight2 the ,russian stones have a better colour and are more beautiful1 but they are less transparent1 and rarely exceed a carat in weight4 ,good chrysolite may be worth from #ajs4 to #b a carat1 and cat's-eye runs from #a to #d a carat1 depending upon the quality4 ,alexandrites meet with a steady demand in ,russia1 and fine stones are scarce2 flawless stones about a carat in weight are worth as much as #cj a carat1 and even quite ordinary stones fetch #d a carat4 ,from ,ceylon1 that interesting home of gems1 have originated some magnificent chrysoberyls1 including a superb chrysolite1 #hj carats in weight1 and another1 a splendid brownish yellow in colour and very even in tint1 and two large alexandrites1 green in daylight and a rich red by night1 weighing #fc and #bh-bc_/cb carats4 ,the finest cut chrysolite existing is probably the one exhibited in the ,mineral ,gallery of the ,british ,museum 7,natural ,history74 ,absolutely flawless and weighing #dc carats1 it was formerly contained in the famous ,hope collection1 and is described on page #ef and figured on ,plate ,,xxi of the catalogue prepared by ,b4 ,hertz1 which was published in #ahci2 the weight there given includes the brilliants and the ring in which it was mounted4 ,it is shown1 about actual size1 in ,plate ,,xxvii1 ,fig4 #ab4 ,a magnificent cat's-eye1 #cee by #ce mm4 in size1 which also formed part of the ,hope collection1 was included in the crown jewels taken from the ,king of ,kandy in #ahae4 ,the crystalline markings in the cut stone are so arranged that the lower half shows an altar overhung by a torch4 ,the stone has been famous in ,ceylon for many ages4 ,it was set in gold with rubies cut en cabochon4 ,two fine ,ceylon alexandrites of exceptional merit1 weighing #db and #bf carats1 are also exhibited in the ,mineral ,gallery of the ,british ,museum 7,natural ,history74 ,the former is illustrated in ,plate ,,xxvii1 ,figs4 #aa1 #ac1 as seen in daylight and in artificial light4 ,,chapter ,,xxviii ,,quartz 7,rock-,crystal1 ,amethyst1 ,citrine1 ,cairngorm1 ,cat's-,eye1 ,tiger's-,eye7 ,although the commonest and1 in its natural form1 the most easily recognizable of mineral substances1 quartz nevertheless holds a not inconspicuous position among gem-stones1 because1 as amethyst 7,plate ,,xxvii1 ,fig4 #g71 it provides stones of the finest violet colour2 moreover1 the yellow quartz 7,plate ,,xxvii1 ,fig4 #e7 so ably vies with the true topaz that it is universally known to jewellers by the name of the latter species1 and is too often confounded with it1 and the lustrous1 limpid rock-crystal even aspires to the local title of #8diamond4' ,for all purposes where a violet or yellow stone is required1 quartz is admirably suited2 it is hard and durable1 and it has the merit1 or possibly to some minds the drawback1 of being moderate in price4 ,despite its comparative lack of #8fire1' rock-crystal might replace paste in rings and buckles with considerable advantage from the point of view of durability4 ,the chatoyant quartz1 especially in the form known as tiger's-eye1 will for beauty bear comparison with the true cat's-eye1 which is a variety of chrysoberyl4 ,except that cat's-eye is cut en cabochon1 quartz is step- or sometimes brilliant-cut4 ,ranking with corundum next to diamond as the simplest in composition of the gem-stones1 quartz is the crystallized form of silica1 oxide of silicon1 corresponding to the formula ,si,o#b4 ,when pure1 it is entirely devoid of the faintest trace of colour and absolutely water-clear4 ,such stones are called rock-crystal1 and it is easy to understand why in early days it was supposed to represent a form of petrified water4 ,it is these brilliant1 transparent stones that are1 when small1 known in many localities as #8diamonds4' ,before the manufacture of glass was discovered and brought to perfection1 rock-crystal was in considerable use for fashioning into cups1 vases1 and so forth4 ,the beautiful tints characterizing quartz are due to the usual metallic oxides4 ,to manganese is given the credit of the superb purple or violet colour of amethyst1 which varies considerably in depth4 ,jewellers are inclined to distinguish the deep-coloured stones with the prefix #8oriental1' but the practice is to be deprecated1 since it might lead to confusion with the true oriental amethyst1 which is a purple sapphire1 one of the rarest varieties of corundum4 ,quartz of a yellow hue is properly called citrine1 but1 as already stated1 jewellers habitually prefer the name #8topaz' for it1 and distinguish the true topaz by the prefix ,brazilian--not a very happy term1 since both the yellow topaz and the yellow quartz occur plentifully in ,brazil4 ,sometimes the yellow quartz is termed occidental1 ,spanish1 or false topaz4 ,stones with a brownish or smoky tinge of yellow are called cairngorm1 or ,scotch topaz4 ,the colour of the yellow stones is doubtless due to a trace of ferric oxide4 ,stones of a smoky brown colour are known as smoky-quartz4 ,rose-quartz1 which is rose-red or pink in colour and hazy in texture1 is comparatively rare2 strange to say1 it has never been found in distinct crystals4 ,the tint1 which may be due to titanium1 is fugitive1 and fades on exposure to strong sunlight4 ,in milky quartz1 as the name suggests1 the interior is so hazy as to impart to the stone a milky appearance4 ,it has frequently happened that quartz has crystallized after the formation of other minerals1 with the result that the latter are found inside it4 ,prase1 or mother-of-emerald1 which at one time was supposed to be the mother-rock of emerald1 is a quartz coloured leek-green by actinolite fibres in the interior4 ,specimens containing hair-like fibres of rutile--the so-called flches d'amour--are common in mineral collections1 and are sometimes to be seen worked4 ,when enclosing a massive1 light-coloured1 fibrous mineral1 the stones have a chatoyant effect1 and display1 when suitably cut1 a fine cat's-eye effect2 in tiger's-eye the enclosed mineral is crocidolite1 an asbestos1 the original blue hue of which has been changed to a fine golden-brown by oxidation4 ,quartz which contains scales of mica1 hematite1 or other flaky mineral has a vivid spangled appearance1 and is known as aventurine2 it has occasionally been employed for brooches or similar articles of jewellery4 ,rainbow-quartz1 or iris1 is a quartz which contains cracks1 the chromatic effect being the result of the interference of light reflected from them2 it has been artificially produced by heating the stone and suddenly cooling it4 ,the name of the species is an old ,german mining term of unknown meaning which has been in general use in all languages since the sixteenth century4 ,amethyst is derived from 1 not drunken1 possibly from a foolish notion that the wearer was exempt from the usual consequences of unrestrained libations4 ,pliny suggests as an alternative explanation that its colour approximates to1 but does not quite reach1 that of wine4 ,aventurine1 from aventura1 an accident1 was first applied to glass spangled with copper1 the effect being said to have been accidentally discovered owing to a number of copper filings falling into a pot of molten glass in a ,venetian factory4 ,illustration3 ,,fig4 #gi4--,quartz ,crystal4 ,quartz belongs to the hexagonal system of crystalline symmetry1 and crystallizes in the familiar six-sided prisms terminated by six inclined1 often triangular1 faces 7,fig4 #gi72 twins are common1 though they are not always obvious from the outward development4 ,in accordance with the symmetry the refraction is double1 and there is one direction of single refraction1 namely1 that parallel to the edge of the prism4 ,the ordinary refractive index has the value #aedd1 and the extraordinary #aeec1 and since the latter is the greater1 the sign of the double refraction is positive4 ,the double refraction is small in amount1 but is large enough to enable the apparent doubling of certain of the opposite edges of a faceted stone to be perceptible when viewed with a lens through the table-facet4 ,the dichroism of the deep-coloured stones is quite distinct4 ,quartz has only about the same amount of colour dispersion as ordinary glass1 and lacks1 therefore1 #8fire4' ,the application of strong heat tends1 as usual1 to weaken or drive off the colour4 ,thus the dense smoky-quartz found in ,spain1 ,brazil1 and elsewhere is converted into stones of a colour varying from light yellow to reddish brown according to the amount and duration of the application4 ,in the case of amethyst the colour is changed to a deep orange1 or entirely driven off if the temperature be high enough4 ,its density is very constant1 varying only from #bfed to #bffj2 the purest stones are the lightest4 ,to it has been assigned the symbol #g on ,mohs's scale of hardness4 ,to physicists quartz is one of the most interesting of minerals because of its power of rotating1 to an extent depending upon the thickness of the section1 the plane of polarization of a beam of light traversing it in a direction parallel to the prism edge4 ,it appears1 moreover1 from a study of the pyro-electric and general physical characters1 that its molecular structure has a helical arrangement1 which1 like all screws1 may have a right- or left-handed character4 ,amethyst is1 in fact1 invariably composed of separate twin individuals1 alternately right- and left-handed2 in some remarkable crystals the section at right angles to the prism edge is composed of triangular sectors1 alternately of different hands and of different tints--purple and white4 ,to the twinning is due the rippled fracture and the feathery inclusions so characteristic of amethyst4 ,besides its use for ornamental purposes1 quartz finds a place as the material for lenses intended for delicate photographic work1 because its transparency to the ultra-violet light is so much greater than that of glass4 ,spectacle lenses made of it are in demand1 because they are not liable to scratches1 and retain1 therefore1 their polish indefinitely4 ,when fused in the oxyhydrogen flame1 quartz becomes a silica glass1 of specific gravity #bb and hardness #e on ,mohs's scale1 which has proved of great service for laboratory ware1 because it withstands sudden and unequal heating without any danger of fracture2 it has also in fine threads been invaluable for delicate torsion work1 because it acquires not the smallest amount of permanent twist1 in this respect being superior to the finest silk threads4 ,clear rock-crystal fetches little more than the cost of the cutting2 citrine and amethyst are worth from #as4 to #es4 a carat1 depending upon the quality and size of the stone2 smoky-quartz is practically valueless2 rose-quartz realizes less than #as4 a carat2 and the value of cat's-eye is also small--only #as4 to #bs4 #f;d4 a carat4 ,tiger's-eye at one time commanded as much as #bes4 a carat1 but the supply exceeded the demand1 with the consequent collapse in the price4 ,beautiful1 brilliant1 and limpid rock-crystal is found in various parts of the world3 in the ,swiss ,alps1 at ,bourg d',oisans in the ,dauphin ,alps1 ,france1 in the famous ,carrara marble1 in the ,marmaros ,comitat of ,hungary1 and in the ,united ,states1 ,brazil1 ,madagascar1 and ,japan4 ,small lustrous stones1 known in their localities as #8,isle of ,wight1' #8,cornish1' or #8,bristol diamonds1' are found in our own country4 ,brazil supplies stones out of which have been cut the clear balls used in crystal-gazing4 ,the finest amethysts come from ,brazil--especially the ,state of ,rio ,grande do ,sul--and from ,uruguay1 ,india1 and the gem-gravels of ,ceylon2 good stones also occur at ,ekaterinburg1 in the ,ural ,mountains4 ,a splendid ,brazilian amethyst1 weighing #ccd carats1 and two ,russian stones--one hexagonal in contour1 weighing #hh carats1 and the other1 a deep purple in colour with a circular table1 weighing #gc carats--are exhibited in the ,british ,museum 7,natural ,history74 ,cairngorm is known from the place of that name in ,banffshire1 ,scotland1 whence fine specimens have emanated2 it is a gem much valued in that country4 ,fine cairngorm has also originated from ,pike's ,peak1 ,colorado4 ,splendid yellow stones have had their birth in the ,states of ,minas ,geraes1 ,so ,paulo1 and ,goyz1 of ,brazil--especially in the last4 ,the fine ,spanish smoky-quartz1 which1 as already stated1 turns yellow on heating1 comes from ,hinojosa1 in the ,province of ,cordova4 ,the delicate rose-quartz is known at ,bodenmais in ,bavaria1 ,paris in ,maine1 ,united ,states1 and ,ekaterinburg in the ,ural ,mountains4 ,the finest cat's-eyes are found in ,india and ,ceylon1 and are high in favour with the natives4 ,greenish stones of an inferior quality are brought from the ,fichtelgebirge in ,bavaria1 and are sold as #8,hungarian cat's-eyes1' despite the fact that no such stone occurs in ,hungary--another instance of jewellers' disdain for accuracy4 ,tiger's-eye occurs in considerable quantity in the neighbourhood of ,griquatown1 ,griqualand ,west1 ,south ,africa4 ,a silicified crocidolite1 in which the blue colour is retained1 comes also from ,salzburg1 and is known as sapphire- or azure-quartz1 or siderite4 ,certain of the pebbles found on the seashore of our coasts1 especially off the ,isle of ,wight and ,north ,wales1 cut into attractive1 clear stones1 more or less yellow in colour2 but examples suitable for the purpose are not so numerous as might be supposed1 and do not reward any casual search4 ,les affaires sont les affaires4 ,the local lapidary1 instead of explaining that the pebbles brought to him are not worth cutting1 finds it more convenient and profitable to substitute for them other1 inferior and badly cut1 stones1 bought by the gross1 or even paste stones2 the customer1 on the other hand1 is contented with a pretty bauble1 and is not grateful for the information that it might have been obtained for a fraction of the sum paid4 ,,chapter ,,xxix ,,chalcedony1 ,,agate1 ,,etc4 ,chalcedony and agate1 and their endless varieties1 are composed mainly of silica1 but the separate individual crystals are so small as to be invisible to the unaided eyesight1 and occasionally are so extremely minute that the structure is almost amorphous4 ,the colour and appearance vary greatly1 depending upon the impurities contained in the stone1 and1 since both have been made a criterion for differentiation of types1 a host of names have come into use1 none of which are susceptible of strict definition4 ,on the whole1 these stones may be divided into two groups3 chalcedony1 in which the structure is concretionary and the colour comparatively uniform1 and agate1 in which the arrangement takes the form of bands1 varying greatly in tint and colour4 ,the refraction1 though double in the individual1 is irregular over the stone as a whole1 and the indices approximate to #aeej4 ,the specific gravity ranges from #bfb to #bfd1 depending upon the impurities present4 ,the degree of hardness is about the same as that of quartz1 namely1 #g on ,mohs's scale4 ,all kinds are more or less porous1 and stones of a dull colour are therefore artificially tinted after being worked4 ,the term chalcedony1 derived from the name of a town in ,asia ,minor1 is usually confined to stones of a greyish tinge4 ,stones artificially coloured an emerald green have been cut and put upon the market as #8emeraldine4' ,carnelian is a clear red chalcedony1 and sard is somewhat similar1 but brownish in tint4 ,chrysoprase is apple-green in colour1 nickel oxide being supposed to be the agent4 ,prase 7cf4 p4 #bdj71 which is a dull leek-green in hue1 may also in part be referred here2 the name comes from 1 a leek4 ,plasma1 which may have the same derivation1 is a brighter leek-green4 ,jasper is a chalcedony coloured blood-red by iron oxide1 while bloodstone is a green chalcedony spotted with jasper2 they are popular stones for signet rings4 ,flint1 an opaque chalcedony1 breaks with a sharp cutting edge1 and was much in request with early man as a tool or a weapon2 its property of giving sparks when struck with steel rendered it invaluable before the invention of matches4 ,hornstone is somewhat similar1 but more brittle1 while chert is a flinty rock4 ,agate1 named after the river ,achates in ,sicily1 where it was found at the time of ,theophrastus1 has a peculiar banded structure1 the bands being usually irregular in shape1 following the configuration of the cavity in which it was formed4 ,moss-agate1 or mocha-stone1 contains moss-like inclusions of some fibrous mineral4 ,onyx is an agate with regular bands1 the layers having sharply different colours2 when black and white1 it has1 in days gone by1 been employed for cameos4 ,sardonyx is similar in structure1 but red and white in colour4 ,agate is used in delicate balances for supporting the steel knife-edges of the balance itself and of the panholders1 and is largely employed--especially when artificially coloured--for umbrella handles and similar articles4 ,chalcedony and agate are found the whole world over1 but ,india1 and particularly ,brazil1 are noted for their fine carnelians and agates4 ,,chapter ,,xxx ,,opal 7,white ,opal1 ,black ,opal1 ,fire-,opal7 ,that opal in early times excited keen admiration is evident from ,pliny's enthusiastic description of these stones3 8,for in them you shall see the burning fire of the carbuncle1 the glorious purple of the amethyst1 the green sea of the emerald1 all glittering together in an incredible mixture of light40 ,during much of last century1 owing to the foolish superstition that ill-luck dogs the footsteps of the wearer1 the species lay under a cloud1 which has even now not quite dispersed1 but exercises a prejudicial effect upon the fortunes of the stone4 ,it has1 however1 recently attracted considerable attention owing to the discovery of the splendid black opals in ,australia2 at one moment black with the darkness of night1 at the next by a chance movement glowing with vivid crimson flame1 such stones may justly be considered the most remarkable in modern jewellery4 ,at the present day opal is divided by jewellers roughly into two main groups3 #8white' 7,plate ,,xxvii1 ,fig4 #f7 and #8black' 7,plate ,,xxvii1 ,fig4 #i71 according as the tint is light or dark1 fire-opal 7,plate ,,xxvii1 ,fig4 #aj7 standing in a separate category4 ,opal differs from the rest of the principal gem-stones in being not a crystalline body1 but a solidified jelly1 and it depends for its attractiveness upon the characteristic play of colour1 known1 in consequence1 as opalescence 7cf4 p4 #ci71 which arises from a peculiarity in the structure4 ,opal is mainly silica1 ,si,o#b1 in composition1 but contains in addition an amount of water varying in precious opal from #f to #aj per cent4 ,as the original jelly cooled1 it became riddled throughout with cracks1 which were afterwards generally filled with opal matter1 containing a different amount of water1 and therefore differing slightly in refractivity from the original substance4 ,the structure not being quite homogeneous1 each crack has the same action upon light as a soap-film1 and gives rise to precisely similar phenomena2 the thinner and more uniform the cracks1 the greater the splendour of the chromatic display1 the particular tint depending upon the direction in which the stone is viewed4 ,the cracks in certain opals were not filled up1 and therefore contain air4 ,such stones appear opaque and devoid of opalescence until plunged into water2 they are consequently known as hydrophane1 from 1 water1 and 1 to make appear4 ,owing to the effect of total-reflection1 light was stopped on the hither side of the cracks before they were filled with water1 which is not far inferior to opal in refractivity2 it is surprising how much water these stones will absorb4 ,opal is colourless when pure1 but is nearly always more or less milky and opaque1 or tinted various dull shades by ferric oxide1 magnesia or1 alumina4 ,the so-called black opal is generally a dark grey or blue1 and very rarely quite black4 ,that the coloration is not due to ordinary absorption1 but to the action of cracks in the stone1 is shown by the fact that the transmitted light is complementary to the reflected light2 the blue opal is1 for instance1 a yellow when held up so that light has passed through it4 ,in many black opals the opalescent material occurs in far too tiny pieces to be cut separately1 and the whole iron-stained matrix is cut and polished and sold under the name #8opal-matrix4' ,the reddish and orange-coloured stones known as fire-opal have pronounced colour and only slight milkiness2 they display the customary opalescence in certain directions4 ,these stones are often faceted1 but otherwise opals are cut en cabochon1 either flat or steep--generally the former in brooches and pendants1 and the latter in rings4 ,opal is somewhat soft1 varying from #e to #f on ,mohs's scale1 and is therefore easily scratched4 ,the specific gravity ranges from #baj to #bbj1 and the refractive index from #addd to #adfd1 the refraction1 of course1 being always single4 ,it is unwise to immerse opals in liquids on account of their porosity4 ,the name opal comes to us through the ,latin opallus1 which was used for the same species as understood by the term at the present day1 but the word has a far older origin1 which has not been traced4 ,the ,romans also called the mineral pderos1 the ,greek form of ,cupid1 a name applied to all rosy stones4 ,the name cacholong1 for the bluish-white porcelain variety1 which is very porous and adheres to the tongue1 is of ,tartar origin2 the stone is highly valued in the ,east4 ,the oldest mines1 which up to quite a recent date were the only extensive deposit of opal known1 were at ,cserwenitsa1 near ,kashau1 in ,hungary4 ,from them in all probability emanated the opals known to the ,romans4 ,the opals from this locality were generally quite small1 and large pieces were rare and commanded high prices4 ,the ,hungary mines1 however1 proved quite unable to compete with the rich fields at ,white ,cliffs1 ,new ,south ,wales1 in spite of the efforts that were made to depreciate and exclude from the market the new stones1 and at the present time few of the opals on the market come from them4 ,as so often happens1 the ,white ,cliffs deposit was discovered by accident4 ,in #ahhi a hunter1 when tracking a wounded kangaroo1 chanced to pick up an attractively coloured opal4 ,the district is so waterless and forbidding that1 but for such a chance1 the opals might have long lain hidden4 ,they occur in seams in deposits of ,cretaceous ,age in a variety of ways1 filling cavities in rocks or sandstones1 or cracks in wood1 or replacing wood1 saurian bones1 and some spiky mineral1 which may have been glauberite4 ,in recent years1 another rich deposit was discovered farther north1 on both sides of the boundary between ,queensland and ,new ,south ,wales4 ,the field is remarkable for the darkness of its opals1 which are called #8black opal' in contradistinction to the lighter-coloured stones previously known4 ,from ,lightning ,ridge in ,new ,south ,wales come stones stained deep black which quite merit the designation black opal4 ,the sandstone in which they are found is rich in iron1 and this is no doubt responsible for the deepness of their tint4 ,mexico is noted for the fire-opal1 which is found at ,esperanza1 ,queretaro1 and ,zimapan2 but other kinds of opal also are found at these places4 ,illustration3 ,,plate ,,xxviii ,,opal ,,mines1 ,,white ,,cliffs1 ,,new ,,south ,,wales ,the price of opal varies greatly1 according to the intrinsic colour and the uniformity and brilliance of the opalescence4 ,common opal can be bought at as low a rate as #as4 a carat1 while black opal ranges from #ajs4 to #h a carat2 but a good dark stone displaying a flaming opalescence commands a fancy figure1 fine stones of this class being exceedingly rare4 ,fire-opal enjoys only a limited popularity now1 though a few years ago it was in some demand2 the price runs from #bs4 to #ajs4 a carat4 ,,chapter ,,xxxi ,,felspar 7,moonstone1 ,sunstone1 ,labradorite1 ,amazon-,stone7 ,though second to none among minerals in scientific interest1 whether regarded from the point of view of their crystalline characters or the important part they play in the formation of rocks1 the group included under the general name felspar occupies but a humble place in jewellery4 ,it consists of three distinct species1 orthoclase1 albite1 and anorthite1 which are silicates of aluminium1 and potassium1 sodium1 or calcium1 corresponding to the formulae ,k,al,si#c,o#h1 ,na,al,si#c,o#h1 and ,ca,al#b,si#b,o#h respectively1 and also of species intermediate in composition between albite and orthoclase1 or albite and anorthite4 ,while differing in crystalline symmetry1 all are characterized by two directions of cleavage which are nearly at right angles to one another4 ,the double refraction1 which is slight in amount1 is biaxial in character and variable in sign4 ,the values of the least and greatest of the indices of refraction range between #aeb and #aec1 and #aec and #aee respectively1 the double refraction at the same time varying from #jjjg to #jjab4 ,the specific gravity lies between #bdh and #bff1 and the hardness ranges between the degrees #f and #g on ,mohs's scale4 ,moonstone 7,plate ,,xxix1 ,fig4 #d71 which is mainly pure orthoclase1 alone is at all common in jewellery4 ,it forms such an admirable contrasting frame for large coloured stones that it deserves greater popularity2 no doubt the cheapness of the stones militates against their proper appreciation4 ,the milky1 bluish opalescence from which they take their name is caused by the reflection of light at the thin twin-lamell of which the structure is composed4 ,they are always cut more or less steeply en cabochon4 ,the finest stones were at one time cut from the felspar that came from the ,st4 ,gothard district in ,switzerland and was in consequence known as adularia from the neighbouring ,adular ,mountains1 somewhat incorrectly1 since none occurs at the latter locality4 ,at the present day practically all the moonstones on the market come from ,ceylon4 ,they run in price from #c to #bj per oz4 7#bh grams74 ,sunstone is a felspar containing flakes of hematite or goethite which impart a spangled bronze appearance to the stones4 ,good material occurs in parts of ,norway4 ,the remarkable sheen of labradorite or blue felspar has its origin in the interference of light at lamellar surfaces in the interior2 the uniformity of the colour over comparatively large areas testifies to the regularity of the lamellar arrangement4 ,the finest specimens were brought from the ,isle of ,st4 ,paul off the coast of ,labrador1 where they were first discovered in #aggj2 large masses also occur on the coast itself4 ,amazon-stone is an opaque green felspar which occurs in the ,ilmen ,mountains1 ,orenburg1 ,russia1 and at ,pike's ,peak1 ,colorado1 ,united ,states4 ,it obtains its name from the ,amazon ,river1 where1 however1 none has ever been found2 there may have been some confusion with a jade or similar stone4 ,occasionally clear colourless felspar has been faceted1 and then closely resembles rock-crystal4 ,a careful determination of the refractive indices and the specific gravity serves to discriminate between them4 ,illustration3 ,,plate ,,xxix #a4 ,,cat',s ,,eye #b4 ,,peridot #c4 ,,peridot #d4 ,,moonstone #e4 ,,hessonite #f4 ,,pyrope #g4 ,,demantoid #h4 ,,almandine #i4 ,,spodumene #aj4 ,,kunzite #aa4 ,,hiddenite #ab4 ,,zircon #ac4 ,,zircon #ad4 ,,zircon #ae4 ,,andalusite #af4 ,,nephrite #ag4 ,,turquoise #ah4 ,,jadeite ,,gem-,,stones ,,chapter ,,xxxii ,,turquoise1 ,,odontolite1 ,,variscite ,of all the opaque stones turquoise 7,plate ,,xxix1 ,fig4 #ag7 alone finds a prominent place in jewellery and can aspire to rank with the precious stones4 ,the colour varies from a sky-blue or a greenish blue to a yellowish green or apple-green4 ,only the former tints1 which are at the same time the rarer1 are in general demand1 and they possess the great advantage of harmonizing with the tint of the gold setting4 ,the blue colours are1 especially in the case of the ,siberian stones1 by no means permanent1 and fade in course of time4 ,turquoise is amorphous and seldom crystalline1 and is therefore somewhat porous2 it should consequently never be immersed in liquids or be contaminated with greasy and dirty matter lest the dreaded change of colour be brought about4 ,the stones are translucent in thin sections1 and a good observation is possible with the refractometer if the back of the stone is flat and polished1 since only the section immediately adjacent to the instrument is concerned2 the refractive index is about #afa4 ,the specific gravity varies from #bge to #bhi4 ,turquoise has a hardness of slightly under #f on ,mohs's scale1 and takes a good polish1 which is fairly durable1 since on account of the comparative opacity of the stones scratches on the surface are not very noticeable4 ,in composition it is a complex phosphate of aluminium and copper1 corresponding to the formula ,cu,o,h4#f;,al7,,oh7#b4;,h#e47,,po#d7d1 with ferric oxide replacing some alumina4 ,the blue colour is due to the copper constituent1 and the predominance of iron may cause the greenish shades2 but the water contained in the stones plays no mean part1 since they turn a dirty green when it is driven off4 ,the faded colour can sometimes be restored by immersion of the stone in ammonia and subsequent application of grease1 but the effect is not lasting4 ,attempts are sometimes made to improve inferior stones by impregnating them with ,berlin blue1 but with only qualified success4 ,turquoises are said to be affected by the perspiration from the skin4 ,the name of the species comes from a ,french word meaning ,turkish1 and arises from the fact that the gem-stone first reached ,europe by way of ,turkey4 ,another1 but less obvious1 suggestion is that it is derived from the ,persian name for the species1 piruzeh4 ,our turquoise and other phosphates of similar appearance were probably known to ,pliny under the three names callais1 callaina1 and callaica4 ,the finest turquoise still comes from the famous mines near ,nishapur in the ,persian province of ,khorassan1 where it was known in very ancient times2 it is found with limonite filling the cracks and cavities in a brecciated porphyritic trachyte4 ,pieces of the turquoise and limonite from here are sometimes cut without removal of the latter1 and sold as #8turquoise-matrix1' when the precious stones are too tiny to be worth separate working4 ,it also occurs at ,serbl in the ,sinai ,peninsula4 ,among the more recent localities may be mentioned ,los ,cerillos ,mountains1 ,new ,mexico2 ,sierra ,nevada1 ,nevada1 where pale blue and green stones are found2 ,san ,bernardino ,county1 ,california1 where again the stones are rather pale2 and ,arizona1 where it occurs in pale greenish-blue stones4 ,some of the stones that have been seen are not the true turquoise but odontolite1 or bone turquoise1 which consists of the teeth and bones of mastodon or other extinct animals1 phosphate of iron being the colouring material4 ,these stones may easily be recognized by their organic structure1 which is clearly visible if viewed with a strong lens or under the microscope4 ,moreover1 odontolite invariably contains some calcium carbonate1 and effervescence takes place if it be touched with hydrochloric acid4 ,turquoise dissolves in hydrochloric acid1 but without effervescence1 and since it contains copper1 a fine blue colour is imparted to the solution by the addition of ammonia4 ,odontolite has a higher specific gravity1 #cj to #ce1 but lower hardness1 #e on ,mohs's scale4 ,variscite1 the hydrated phosphate of aluminium1 corresponding to the formula ,al,p,o#d #b;,h#b,o1 is found in masses resembling a greenish turquoise1 but it is much softer1 being only #d on ,mohs's scale4 ,the specific gravity is #bee4 ,round nodular masses of variscite are found in ,utah4 ,,chapter ,,xxxiii ,,jade ,though not usually accounted precious among ,european nations or in ,western civilization in general1 jade was held in extraordinary esteem by primitive man1 and was fashioned by him into ornaments and utensils1 often of considerable beauty1 and even at the present day it ranks among the ,chinese and ,japanese peoples above all precious stones2 indeed1 the ,chinese word ,yu and the ,japanese words ,giyuku or ,tama signify both jade and precious stones in general4 ,according to the ,chinese1 jade is the prototype of all gems1 and unites in itself the five cardinal virtues--,jin1 charity2 ,gi1 modesty2 ,yu1 courage2 ,ketsu1 justice2 and ,chi1 wisdom4 ,when powdered and mixed with water1 it is supposed to be a powerful remedy for all kinds of internal disorders1 to strengthen the frame and prevent fatigue1 to prolong life1 and1 if taken in sufficient quantity just before death1 to prevent decomposition4 ,jade is a general term that includes properly two distinct mineral species1 nephrite or greenstone1 and jadeite1 which are very similar in appearance1 both being fibrous and tough in texture1 and more or less greenish in colour2 but it is also applied to other species such as saussurite1 californite1 bowenite1 and plasma1 which have somewhat similar characters4 ,the word jade is a corruption of the ,spanish pietra di hijada1 kidney-stone1 in allusion to its supposed efficacy in diseases of that organ4 ,nephrite or greenstone 7,plate ,,xxix1 ,fig4 #af7 is the commoner of the two jades4 ,it is closely allied to the mineral hornblende1 a silicate of magnesium1 iron1 and calcium corresponding to the formula ,ca7,mg1,fe7#c7,si,o#c7d1 the magnesia being replaceable by ferrous oxide4 ,microscopic examination shows that the structure consists of innumerable independent fibres foliated or matted together1 the former character giving rise to a slaty and the latter to a horny appearance in the stone as seen by the unaided eye4 ,the colour varies from grey to leaf- and dark-green1 the tint deepening as the relative amount of iron in the composition increases1 and brown tints result from the oxidation of the iron along cracks in the stone4 ,the hardness is #f on ,mohs's scale2 nephrite is therefore about as hard as ordinary glass and softer than quartz4 ,when polished1 it always acquires a greasy lustre4 ,the specific gravity ranges from #bi to #ca4 ,the least and greatest of the principal refractive indices are #afjf and #afcb respectively1 the double refraction being biaxial and negative2 the coloured fibres also display dichroism4 ,all these differential effects are1 however1 masked in the stone because of the irregularity of the aggregation4 ,nephrite is fusible before the blowpipe1 but only with difficulty4 ,its name is derived from the ,greek word 1 kidney1 the allusion being the same as for jade4 ,many of the prehistoric implements found in ,mexico and in the ,swiss ,lake ,habitations are composed of nephrite1 but it is uncertain where the mineral was obtained4 ,much of the material used by the ,chinese at the present time comes from spots near the southern boundary of ,eastern ,turkestan1 especially in the valleys of the rivers ,karakash and ,yarkand in the ,kwen ,lun range of mountains2 it is also found farther north at the river ,kashgar4 ,it occurs in various provinces of ,china1 namely1 ,shensi1 ,kwei ,chau1 ,kwang ,tung1 ,yunnan1 and ,manchuria4 ,gigantic waterworn boulders have been found in the ,government of ,irkutsk1 near ,lake ,baikal1 in eastern ,siberia1 the first discovery being made in the bed of the ,onot stream by the explorer and prospector ,j4 ,p4 ,alibert1 in #ahej4 ,a large boulder of this kind1 weighing over half a ton 7#aaef lb41 or #ebde kg471 is exhibited in the ,mineral ,gallery of the ,british ,museum 7,natural ,history74 ,an enormous mass1 weighing over #b tons 7#dgah lb41 or #badj kg471 was discovered at ,jordansmhl1 ,silesia1 by ,dr4 ,g4 ,f4 ,kunz1 and is now in the magnificent collection of jade formed by ,mr4 ,heber ,r4 ,bishop4 ,beautiful greenstone occurs in ,new ,zealand1 particularly in the ,middle ,island4 ,the ,maoris have long used it for various useful and ornamental purposes1 the most common being indicated by their general name for the species1 punamu1 axe-stone2 kawakawa is the ordinary green variety1 a fine section of which is shown on the wall of the ,mineral ,gallery of the ,british ,museum 7,natural ,history71 while inanga1 a grey variety1 and kahurangi1 a pale-green and translucent variety1 are rare and highly prized4 ,jadeite 7,plate ,,xxix1 ,fig4 #ah7 is by far the rarer of the two jades1 and is the choicest gem with the ,chinese4 ,in composition it is a silicate of sodium and aluminium with the formula ,na,al7,si,o#c7b1 corresponding to the lithium mineral spodumene 7p4 #bfe74 ,it has the same toughness and greasy lustre as nephrite1 but is harder1 being represented by the symbol #g on ,mohs's scale1 and thus only slightly1 if at all1 softer than quartz4 ,the other characters are also higher2 the specific gravity is about #ccd1 and the least and greatest of the principal refractive indices are #aff and #afh1 the double refraction being biaxial and negative4 ,the colour varies from white to almost an emerald green1 the latter being especially prized1 and often the green colour runs in streaks through the white4 ,jadeite fuses readily before the blowpipe to blebby glass1 more easily than is the case with nephrite4 ,the finest jadeite comes from the ,mogaung district in ,upper ,burma1 where it is found in boulders and also with albite in dykes in a dark-green serpentine4 ,the export trade to ,china1 which absorbs practically the whole of the output1 is exceedingly valuable1 and realizes nearly as much as the produce of the ruby mines4 ,jadeite is also found in the ,shensi and ,yunnan provinces of ,china1 and in ,tibet4 ,a few words may be said about the other jade-like minerals4 ,saussurite1 which is named after ,h4 ,b4 de ,saussure1 has resulted from the decomposition of a felspar1 and is nearly akin to the mineral zoisite4 ,it has the customary toughness of structure1 and is greenish grey to white in colour4 ,its specific gravity is about #cb1 and hardness #f to #g on ,mohs's scale4 ,it occurs near ,lake ,geneva4 ,bowenite is a green serpentine 7p4 #bhi7 which is found at ,smithfield1 ,rhode ,island1 ,u4,s4,a41 and in ,new ,zealand and ,afghanistan4 ,californite and plasma are compact varieties of idocrase 7p4 #bge7 and chalcedony 7p4 #bdg7 respectively4 ,verdite is a stone of rich green colour which is found in the form of large boulders in the ,north ,kaap ,river1 ,south ,africa2 it is composed of green mica 7fuchsite7 and some clayey matter4 ,jade has of recent years been imitated in glass1 but the latter is recognizable by its vitreous lustre and inferior hardness1 and sooner or later by its frangibility4 ,,chapter ,,xxxiv ,,spodumene1 ,,iolite1 ,,benitoite ,,spodumene 7,kunzite1 ,hiddenite7 ,till a few years ago scarcely known outside the ranks of mineralogists1 spodumene suddenly leaped into notice in #aijc upon the discovery of the lovely lilac-coloured stones 7,plate ,,xxix1 ,fig4 #aj7 at ,pala1 ,san ,diego ,county1 ,california2 they shortly afterwards received the name kunzite after the well-known expert in gems1 ,dr4 ,g4 ,f4 ,kunz4 ,the stones were found here in a pegmatite dyke1 and were of all shades1 ranging from pale pink to deep lilac1 and at times as much as #aej carats in weight4 ,paler kunzite occurs with beryl and tourmaline at ,coahuila ,mountain in ,riverside ,county1 ,california1 and colourless stones have recently come to light in ,madagascar4 ,kunzite is remarkable for its wonderful dichroism2 the beautiful violet tint that springs out in one direction comes with greater surprise because of the uninteresting yellowish tints in other directions4 ,unlike spodumene in general1 kunzite is phosphorescent under the influence of radium4 ,the emerald-green variety 7,plate ,,xxix1 ,fig4 #aa71 named hiddenite after ,mr4 ,w4 ,e4 ,hidden1 who discovered in #ahha the only known occurrence1 in ,alexander ,county1 ,north ,carolina1 would no doubt have become popular had the supply of material not been so very limited2 few stones were found1 and the variety has never come to light elsewhere4 ,the colour is supposed to be due to chromic acid4 ,hiddenite being also dichroic1 the tint varies with the direction4 ,spodumene is ordinarily rather a pale yellowish in hue1 and1 as its name 7which is derived from 1 ash-coloured7 suggests1 is not very attractive4 ,clear1 lemon-yellow stones 7,plate ,,xxix1 ,fig4 #i7 are found in ,brazil and ,madagascar4 ,the species is interesting scientifically because it contains the rare element lithium2 it is a silicate of aluminium and lithium1 corresponding to the formula ,li,al7,si,o#c7b4 ,the double refraction is biaxial in character and positive in sign1 the least and greatest of the refractive indices being #affj and #afge2 the specific gravity is #cahe1 and hardness #f to #g on ,mohs's scale4 ,spodumene has an easy cleavage1 and the cut stones call therefore for careful handling1 lest they be flawed or fractured4 ,two faceted stones1 a beautiful kunzite and a fine hiddenite1 weighing #fj and #b carats respectively1 are exhibited in the ,british ,museum 7,natural ,history74 ,,iolite ,known also by various other names--cordierite1 dichroite1 and water-sapphire 7saphire d'eau7--this species owes its interest to the remarkable dichroism characterizing it1 the principal colours--smoky-blue and yellowish white--being in such contrast as to be obvious to the unaided eye4 ,the stones that are usually worked have intrinsically a smoky-blue colour1 and are found in waterworn masses in the river-gravels of ,ceylon1 whence is the origin of the name water-sapphire4 ,iolite1 from 1 violet1 and 1 stone1 refers to the colour2 cordierite is named after ,cordier1 a ,french geologist1 who first studied the crystallography of the species2 and dichroite1 of course1 alludes to the most prominent character of the species4 ,iolite is a silicate of aluminium and of magnesium and iron corresponding to the formula ,h#b7,mg1,fe7#d;,al#h,si#aj,o#cg4 ,the double refraction is small in amount1 biaxial in character1 and negative in sign1 the least and greatest of the refractive indices being #aedc and #aeea2 the specific gravity is #bfc1 and hardness #g on ,mohs's scale4 ,iolite1 if used1 is worked and polished2 it is seldom faceted4 ,a large worked piece1 weighing #agg grams1 which was formerly in the ,hawkins ,collection1 is exhibited in the ,british ,museum 7,natural ,history74 ,,benitoite ,the babe among gem-stones1 benitoite first saw the light of day a few years ago1 early in #aijg4 ,it occurs with the rare mineral neptunite1 which was previously known only from ,greenland1 in narrow veins of natrolite in ,diablo ,range near the head-waters of the ,san ,benito ,river1 ,san ,benito ,county1 ,california4 ,despite careful search the species has not been found except within the original restricted area4 ,to science it is interesting both because of its composition1 a silico-titanate of barium1 corresponding to the formula ,ba,ti,si#c,o#i1 and because its crystals belong to a class of crystalline symmetry which has hitherto not been represented among minerals4 ,the double refraction is uniaxial1 and since the ordinary index of refraction is #ageg and the extraordinary #ahjd1 it is positive in sign and large in amount1 namely1 #jjdg4 ,the stones are characterized by strong dichroism1 the colour corresponding to the ordinary ray being white1 and to the extraordinary greenish blue to indigo depending upon the tint of the stone4 ,to obtain the best effect the stone must therefore be cut with the table-facet parallel to the crystallographic axis4 ,the specific gravity is #cfe1 and hardness #f on ,mohs's scale4 ,when first discovered the species was supposed to be sapphire1 and many stones were cut and sold as such4 ,it is1 however1 much softer than sapphire1 and is readily distinguished by its optical characters1 since it possesses greater double refraction and of differing sign1 so that1 when tested with the refractometer1 the shadow-edge corresponding to the lower index of refraction remains fixed in the case of benitoite1 whereas the contrary happens with sapphire4 ,benitoite also1 unlike sapphire1 fuses easily to a transparent glass4 ,its blue colour1 which is supposed to be due to a small amount of free titanic acid present1 appears to be stable4 ,several stones as large as #a to #b carats in weight have been found4 ,the largest of all1 perfectly flawless1 weighs just over #g carats1 and is remarkable because it is about three times the next largest in point of weight2 it is the property of ,mr4 ,g4 ,eacret1 of ,san ,francisco4 ,,chapter ,,xxxv ,,euclase1 ,,phenakite1 ,,beryllonite ,,euclase ,this species comes near beryl in chemical composition1 being a silicate of aluminium and beryllium corresponding to the formula ,be7,al,o,h7,si,o#d1 and closely resembles aquamarine in colour and appearance when cut4 ,owing to the rarity of the mineral good specimens command high prices for museum collections1 and it is seldom worth while cutting it for jewellery4 ,it derives its name from its easy cleavage1 easily1 and fracture4 ,the double refraction is biaxial in character and positive in sign1 the least and greatest of the refractive indices being #afea and #afgj respectively2 the specific gravity is #cjg1 and the hardness #g on ,mohs's scale4 ,the colour is usually a sea-green1 but sometimes blue4 ,euclase occurs with topaz at the rich mineral district of ,minas ,novas1 ,minas ,geraes1 ,brazil1 and has also been found in the ,ural district1 ,russia4 ,,phenakite ,another beryllium mineral1 phenakite owes its name to the frequency with which it has been mistaken for quartz1 being derived from 1 deceiver4 ,the clear1 colourless crystals1 somewhat complex in form1 have at times been cut1 but they lack #8fire1' and despite their brilliant lustre meet with little demand4 ,the composition is a silicate of beryllium corresponding to the formula ,be#b,si,o#d4 ,the double refraction is uniaxial1 and since the ordinary1 #afeb1 is less than the extraordinary index1 #affg1 it is positive in sign2 the specific gravity is #bii1 and the hardness is almost equal to that of topaz1 being about #g to #h on ,mohs's scale4 ,fine stones have long been known near ,ekaterinburg in the ,ural ,mountains1 and have recently been discovered in ,brazil4 ,,beryllonite ,as its name suggests1 this mineral also contains beryllium1 being a soda phosphate corresponding to the formula ,na,be,p,o#d4 ,clear1 colourless stones1 which occur at ,stoneham1 ,maine1 ,u4,s4,a41 have been cut1 but the lack of #8fire1' the easy cleavage1 and comparative softness1 the symbol being #e on ,mohs's scale1 unfit it for use in jewellery4 ,the double refraction is biaxial in character and negative in sign1 the least and the greatest of the refractive indices being #aeec and #aefe respectively4 ,,chapter ,,xxxvi ,,enstatite1 ,,diopside1 ,,kyanite1 ,,andalusite1 ,,idocrase1 ,,epidote1 ,,sphene1 ,,axinite1 ,,prehnite1 ,,apatite1 ,,dioptase ,,enstatite 7#8,green ,garnet'7 ,the small green stones which accompany the diamond in ,south ,africa have been cut and put on the market as #8green garnet4' ,they are1 however1 in no way connected with garnet1 but belong to a mineral species called enstatite1 which is a silicate of magnesium corresponding to the formula ,mg,si,o#c2 the green colour is due to a small amount of ferrous oxide which replaces magnesia4 ,the double refraction is biaxial in character and positive in sign1 the least and greatest of the refractive indices being #affe and #afgd respectively2 the specific gravity ranges from #caj to #cac1 and the hardness is only about #e on ,mohs's scale4 ,the dichroism is perceptible1 the twin-colours being yellowish and green1 and1 as usual1 is more pronounced the deeper the colour of the stone4 ,there is also a good cleavage in two different directions4 ,with increasing percentage amount of iron enstatite passes into hypersthene4 ,the colour becomes a dark brownish green1 and an increase takes place in the physical constants1 the least and greatest of the refractive indices attaining to #afib and #agje respectively1 and the specific gravity ranging from #cd to #ce4 ,hypersthene is never sufficiently transparent for faceting1 but when spangled with small scales of brookite it is sometimes cut en cabochon4 ,the name enstatite is derived from 1 an opponent1 referring to the infusibility of the mineral before the blowpipe1 and hypersthene comes from 1 very tough4 ,an altered enstatite1 leek-green in colour and with nearly the composition of serpentine 7p4 #bhi71 has been cut en cabochon4 ,it has much lower specific gravity1 only #bf1 and lower hardness1 #c to #d on ,mohs's scale4 ,it is named bastite from ,baste in the ,harz ,mountains1 where it was first discovered4 ,,diopside ,this species1 which is also known as malacolite and alalite1 provides stones of a leaf-green colour which have occasionally been cut4 ,it is a silicate of calcium and magnesium corresponding to the formula ,mg,ca7,si,o#c7b1 but usually contains in place of magnesia some ferrous oxide1 to which it owes its colour2 with increase in the percentage amount of iron the colour deepens and the physical constants change4 ,the double refraction is large in amount1 #jjbh1 biaxial in character1 and positive in sign4 ,the least and greatest of the refractive indices corresponding to the stones suitable for jewellery range about #afga and #afii respectively1 but they may be as high as #agcb and #agej in the two cases4 ,the specific gravity varies from #cbj to #cch1 and the hardness from #e to #f on ,mohs's scale4 ,dichroism is noticeable in deep-coloured stones1 but is not very marked4 ,the name diopside comes from 1 double1 and 1 appearance1 in allusion to the effect resulting from the double refraction2 malacolite is derived from 1 soft1 because the mineral is softer than the felspar associated with it2 and alalite is named after the principal locality1 ,ala ,valley1 ,piedmont1 ,italy4 ,,kyanite ,kyanite1 also known as disthene1 is interesting for two reasons4 ,its structure is so grained in character that the hardness varies in the same stone from #e to #g on ,mohs's scale2 it can therefore be scratched by a knife in some directions1 but not in others 7p4 #gi74 ,it has the same chemical composition as andalusite1 both being silicates of aluminium corresponding to the formula ,al#b,si,o#e1 but possesses very different physical characters1 a fact which shows how large a share the molecular grouping has in determining the aspect of crystallized substances4 ,it is biaxial with small negative double refraction1 the least and greatest of the refractive indices being #agb and #agc respectively2 the specific gravity is #cfa4 ,it occurs in sky-blue prismatic crystals1 whitish at the edges1 in schist near ,st4 ,gothard1 ,switzerland4 ,it is seldom cut4 ,kyanite is derived from its colour1 blue1 and disthene1 from its variable hardness1 1 twice1 and 1 strong4 ,,andalusite ,andalusite bears no resemblance whatever to kyanite1 although1 as has been stated above1 the composition of the two species is essentially the same4 ,it is usually light bottle-green in colour1 and more rarely brown and reddish4 ,its extreme dichroism is its most remarkable character1 the twin colours being olive-green and red4 ,the reddish gleams that are reflected from the interior are in sharp contrast with the general colour of the stone1 and impart to it a weird effect 7,plate ,,xxix1 ,fig4 #ae74 ,cut stones are often confused with tourmalines1 and can1 indeed1 only be distinguished from the latter with certainty by noting on the refractometer the smaller amount of double refraction and the difference in its character4 ,the least and greatest of the refractive indices are #afb and #afdc respectively1 and the double refraction1 #jjaa1 about half that of tourmaline1 is biaxial and negative2 the specific gravity is #cah1 and hardness #g on ,mohs's scale4 ,good stones are found at ,minas ,novas1 ,minas ,geraes1 ,brazil1 and in the gem-gravels of ,ceylon4 ,it was first known from the province of ,andalusia1 ,spain1 whence is the origin of its name4 ,,idocrase 7,vesuvianite1 ,californite7 ,idocrase1 also known as vesuvianite1 is occasionally found in the form of transparent1 leaf-green1 and yellowish-brown stones which1 when cut1 may be mistaken for diopside and epidote respectively1 but are distinguishable from both by the extreme smallness of their double refraction4 ,californite is a compact variety which has all the appearances of a jade2 its colour is green1 or nearly colourless with green streaks4 ,in composition idocrase is a silicate of aluminium and calcium1 the precise formula of which is uncertain1 but may be-- 7,ca1,mn1,mg1,fe7#b7;,al1,fe77,,oh1,f7,s i#b,o#g4 ,the double refraction1 which is uniaxial in character and negative in sign1 may be less than #jjja1 and never exceeds #jjjf1 so that it is not easily detected with the refractometer1 even in sodium light4 ,the refractive indices vary enormously in value1 from #agjb to #agbf for the ordinary1 and from #agjf to #agcb for the extraordinary ray4 ,the specific gravity varies from #cce to #cde1 and the hardness is about #f on ,mohs's scale4 ,the name idocrase1 from 1 form1 and 1 mixture1 was assigned to the species by ,hay1 but his reasons have little meaning at the present day4 ,the other names are taken from the localities where the species and the variety were first discovered4 ,bright1 green crystals come from ,russia1 and also from ,ala ,valley1 ,piedmont1 and ,mount ,vesuvius1 ,italy4 ,californite is found in large masses in ,siskiyon and ,fresno ,counties1 ,california4 ,,epidote 7,pistacite7 ,epidote often possesses a peculiar shade of yellowish green1 similar to that of the pistachio-nut--hence the origin of its alternative name--which is unique among minerals1 though scarcely pleasing enough to recommend it to general taste4 ,its ready cleavage renders it liable to flaws2 nevertheless1 it is occasionally faceted4 ,the name epidote1 from 1 increase1 was given to it by ,hay1 but not on very precise crystallographical grounds4 ,in composition this species is a silicate of calcium and aluminium1 with some ferric oxide in place of alumina1 corresponding to the complex formula1 ,ca#b7;,al1,fe7#b7;,al1,fe7,,oh7,si,o#d7 c4 ,it occurs in monoclinic1 prismatic crystals richly endowed with natural faces4 ,the colour deepens with increase in the percentage amount of iron1 and the stones become almost opaque4 ,the double refraction is large in amount1 #jjca1 biaxial in character1 and negative in sign4 ,the dichroism is conspicuous in transparent stones1 the twin-tints corresponding to the principal optical directions being green1 brown1 and yellow4 ,the values of the least and greatest of the refractive indices given by transparent stones are #agce and #agff respectively2 the specific gravity varies from #cbe to #cej1 and the hardness from #f to #g on ,mohs's scale4 ,transparent crystals have come from ,knappenwand1 ,untersulzbachtal1 ,salzburg1 ,austria2 ,traversella1 ,piedmont1 ,italy2 and ,arendal1 ,nedens1 ,norway4 ,magnificent1 but very dark1 crystals were discovered about ten years ago on ,prince of ,wales ,island1 ,alaska4 ,,sphene 7,titanite7 ,the clear1 green1 yellow1 or brownish stones provided by this species would be welcomed in jewellery because of their brilliant and almost adamantine lustre1 but1 unfortunately1 they are too soft to withstand much wear1 the hardness being only #e on ,mohs's scale4 ,in composition sphene is a silico-titanate of calcium corresponding to the formula ,ca,ti,si,o#e1 and in this respect comes near the recently discovered gem-stone1 benitoite4 ,the refractive indices lie outside the range of the refractometer1 the values of the least and the greatest of the refractive indices varying from #ahhh and #aiag to #aiad and #bjec respectively4 ,it is to this high refraction that it owes its brilliant lustre4 ,the double refraction1 which is biaxial in character and positive in sign1 is so large that the apparent doubling of the opposite edges of a cut stone when viewed through one of the faces is obvious to the unaided eye 7cf4 p4 #da74 ,cut stones have additional interest on account of the vivid dichroism displayed1 the twin-tints1 colourless1 yellow1 and reddish yellow1 corresponding to the three principal optical directions1 being in strong contrast4 ,the specific gravity ranges from #cce to #cde4 ,the negative test with the refractometer 7cf p4 #bf71 the softness1 and the large amount of double refraction suffice to distinguish this species from gem-stones of similar appearance4 ,the name sphene1 from 1 wedge1 alludes to the shape of the natural crystals4 ,the alternative name is obviously due to the fact that the species contains titanium4 ,good stones have come from the ,st4 ,gothard district1 ,switzerland4 ,,axinite ,called axinite from the shape of its crystals--1 axe--this species supplies small1 clear1 clove-brown1 honey-yellow1 and violet stones which can be cut for those who care for a stone out of the ordinary4 ,the composition is a boro-silicate of aluminium and calcium1 with varying amounts of iron and manganese1 corresponding to the formula 7,ca1,fe7#c;,al#b7;,b4,,oh7,si#d,o#ae4 ,axinite is interesting on account of its strong dichroism1 the twin-tints corresponding to the principal optical directions being violet1 brown1 and green4 ,the double refraction is biaxial in character and negative in sign1 the least and greatest of the refractive indices being #afgd and #afhd2 the specific gravity is #cbh1 and hardness about #f to #g1 or rather under that of quartz4 ,the best examples have been found at ,st4 ,cristophe1 ,bourg d',oisans1 in the ,dauphin1 ,france4 ,violet axinite is a novelty that has come within recent years from ,rosebery1 ,montagu ,county1 ,tasmania4 ,,prehnite ,this species1 which is named after its discoverer1 ,colonel ,prehn1 is found in nodular1 yellow and oil-green stones1 of which the latter have very occasionally been cut4 ,it is a little soft1 the hardness being only #f on ,mohs's scale4 ,the double refraction is large in amount1 #jjcc1 biaxial in character1 and positive in sign1 the least and the greatest of the refractive indices being #afaf and #afdi respectively2 the specific gravity varies from #bha to #bie4 ,in composition prehnite is a silicate of aluminium and calcium corresponding to the formula ,h#b;,ca#b;,al#b7,si,o#d7c4 ,the best material has been found at ,st4 ,cristophe1 ,bourg d',oisans1 ,dauphin1 ,france4 ,,apatite ,this interesting mineral is found occasionally in attractive green1 blue1 or violet stones1 but is unfortunately too soft for extensive use in jewellery1 the hardness being only #e on ,mohs's scale4 ,in composition it is a fluo-chloro-phosphate of calcium1 corresponding to the formula ,ca#d;,ca7,f1,cl77,,po#d7c4 ,when pure1 it is devoid of colour1 the tints being due to the presence of small amounts of tinctorial agents4 ,the double refraction is uniaxial in character and negative in sign1 the ordinary index being #afdb and the extraordinary #afdf2 the specific gravity varies from #cag to #cbc4 ,the dichroism is usually feeble1 but sometimes is strong2 for instance1 in the stones from the ,burma ruby mines 7yellow1 blue-green74 ,a cut stone might be mistaken for tourmaline1 but is distinguished by its softness1 or1 when tested on the refractometer1 by its inferior double refraction4 ,it received its name from 1 deceive1 because it was wrongly assigned to at least half a dozen different species in early days4 ,moroxite is a name sometimes given to blue-green apatite4 ,beautiful violet stones are found at ,ehrenfriedersdorf1 ,saxony2 ,schlaggenwald1 ,bohemia2 and ,mount ,apatite1 ,auburn1 ,androscoggin ,county1 ,maine1 ,u4,s4,a42 and blue stones come from ,ceylon4 ,,dioptase ,though of a pretty1 emerald-green colour1 dioptase has never been found in large enough crystals for gem purposes1 and it is1 moreover1 rather soft1 the hardness being only #e on ,mohs's scale1 and has an easy cleavage4 ,in composition it is a hydrous silicate of copper corresponding to the formula ,cu,h#b,si,o#d4 ,the double refraction1 which is large in amount1 is uniaxial in character1 and positive in sign1 the ordinary refractive index being #affg and the extraordinary #agbc4 ,its colour and softness distinguish it from peridot or diopside1 which have about the same refractivity4 ,the name was assigned to the species by ,hay1 from 1 through1 and 1 see1 because the cleavage directions were distinguishable by looking through the stone4 ,dioptase has been found near ,altyn-,tbe in the ,kirghese ,steppes1 at ,rezbnya in ,hungary1 and ,copiapo in ,chili1 and at the mine ,mindouli1 near ,comba1 in the ,french ,congo4 ,,chapter ,,xxxvii ,,cassiterite1 ,,anatase1 ,,pyrites1 ,,hematite ,,cassiterite ,though usually opaque1 this oxide of tin1 corresponding to the formula ,sn,o#b1 has occasionally1 but very rarely1 been found in small1 transparent1 yellow and reddish stones suitable for cutting4 ,the lustre is adamantine4 ,the refraction is uniaxial in character and positive in sign1 the ordinary index being #aiig and extraordinary #bjic4 ,the specific gravity is high1 ranging from #fh to #ga4 ,the hardness is on the whole less than that of quartz1 being about #f to #g on ,mohs's scale4 ,,anatase ,this mineral1 which is one of the three crystallized forms of titanium oxide1 ,ti,o#b1 occurs often in small1 brown1 transparent stones which occasionally find their way into the market4 ,the lustre is adamantine4 ,the refraction is uniaxial in character and negative in sign1 the extraordinary index being #bdic and ordinary #beed4 ,the specific gravity varies from #chb to #cie1 and the hardness is about #e to #f on ,mohs's scale4 ,,pyrites1 ,,hematite ,these two metallic minerals were employed in ancient jewellery4 ,the former1 sulphide of iron1 ,fe,s#b1 is brass-yellow in colour1 and has a specific gravity #eb1 and hardness #f on ,mohs's scale4 ,it is found1 when fresh1 in brilliant cubes4 ,the latter1 oxide of iron1 ,fe#b,o#c1 has a black metallic lustre1 but1 when powdered1 is red in colour--a mode of distinguishing it from other minerals of similar appearance4 ,its specific gravity is #ec1 and hardness #f on ,mohs's scale4 ,in modern times it has been cut in spherical form to imitate black pearls1 but can easily be recognized by its greater density and hardness4 ,hematite is used for signet stones1 often with an intaglio engraving4 ,,chapter ,,xxxviii ,,obsidian1 ,,moldavite ,two forms of natural glass have been employed for ornamental purposes4 ,obsidian results from the solidification without crystallization of lava1 and corresponds in composition to a granite4 ,the structure is seldom clear and transparent1 and usually contains inclusions or streaks4 ,the colour is in the mass jet-black1 but smoky in thin fragments1 and occasionally greenish4 ,its property of breaking with a keen cutting edge1 in the same way as ordinary glass1 rendered it of extreme utility to primitive man1 who was ignorant of the artificial substance4 ,the refraction is1 of course1 single1 and the refractive index approximates to #aej4 ,the specific gravity varies from #bc to #be4 ,the hardness is #e on ,mohs's scale1 the same as ordinary glass4 ,obsidian is obtained wherever there has been volcanic activity4 ,vast mines of great antiquity exist in the ,state of ,hidalgo1 ,mexico4 ,moldavite1 which differs in no respect from ordinary green bottle-glass1 is of interest on account of its problematical origin4 ,its occurrence in various parts of ,bohemia and ,moravia cannot be explained as the result of volcanic agency4 ,it may possibly be the product of old and forgotten glass factories which at one time existed on the site4 ,even meteorites have been suggested as the source4 ,the physical characters are the same as those of ordinary glass3 refraction single1 index #aea2 specific gravity #bej and hardness #e on ,mohs's scale4 ,moldavite also passes under the names of bottle-stone1 or water-chrysolite4 ,a natural glass of the same character has been found in water-worn fragments in ,ceylon1 and has been sold as peridot1 which it resembles in colour1 but is readily distinguished from it by its very different physical properties4 ,,part ,,ii--,,section ,c ,,ornamental ,,stones ,,chapter ,,xxxix ,,fluor1 ,,lapis ,,lazuli1 ,,sodalite1 ,,violane1 ,,rhodonite1 ,,azurite1 ,,malachite1 ,,thulite1 ,,marble1 ,,apophyllite1 ,,chrysocolla1 ,,steatite ,,or ,,soapstone1 ,,meerschaum1 ,,serpentine ,space will not permit of more than a few words concerning the more prominent of the numerous mineral species which are employed for ornamental purposes in articles of virtu or in architecture1 but which for various reasons cannot take rank as gem-stones4 ,fluor1 a beautiful mineral which is found in its greatest perfection in ,england1 has enjoyed well-deserved popularity when worked into vases or other articles4 ,the finest material1 deep purple in colour1 known as #8,blue ,john1' came from ,derbyshire1 but the supply is now exhausted4 ,the crystallized examples1 from ,durham1 ,devonshire1 and ,cornwall1 form some of the most attractive of museum specimens4 ,the crystals take the shape of cubes1 often twinned1 and have an easy octahedral cleavage4 ,the refraction is single1 the index being #adcc4 ,fluor is noted for its property of appearing of differing colour by reflected and transmitted light1 and the phenomenon is in consequence known as fluorescence4 ,the specific gravity is #cah1 and the hardness #d on ,mohs's scale4 ,owing to its low refraction and softness1 fluor is not suitable for jewellery4 ,clear colourless material is in demand for particular lenses of microscope objectives4 ,the lovely blue stone known as lapis lazuli has since the earliest times been applied to all kinds of decorative purposes1 for mosaic and inlaid work and as the material for vases1 boxes1 and so on1 and was the original sapphire of the ancients4 ,when ground to powder it furnishes a fine blue paint1 but it has now been entirely superseded for this purpose by an artificial product4 ,although to the eye so homogeneous and uniform in structure1 lapis lazuli has been shown by microscopic examination to be composed of calcite coloured by three blue minerals in varying proportions4 ,all three belong to the cubic class of symmetry1 and are mainly soda aluminium silicates in composition2 their hardness varies from #e to #f on ,mohs's scale4 ,lazurite1 ,na#d7,na,s#c4;,al7,al#b,si#c,o#ab1 has specific gravity varying from #bch to #bde1 and hardness about #e to #e2 haynite1 7,na#b1;,ca7#b7,na,s,o#d1;,al7,al#b,si#c ,o#ab1 is about the same in specific gravity1 #bd to #be1 but slightly harder1 #e to #f2 while sodalite1 ,na#d7;,al,cl7,al#b,si#c,o#ab1 is the lightest in density1 #bad to #bcj1 with hardness #e to #f1 and has a refractive index #adhc4 ,by far the oldest mines are in the ,badakshan district of ,afghanistan1 a few miles above ,firgamu in the valley of the ,kokcha1 a branch of the ,oxus1 where ruby and spinel are found4 ,it is also found at the southern end of ,lake ,baikal1 ,siberia1 and in the ,chilian ,andes4 ,sodalite occurs in beautiful blue masses at ,dungannon1 ,hastings ,county1 ,ontario1 ,canada1 and at ,litchfield1 ,maine1 ,u4,s4,a4 ,they make excellent polished stones4 ,violane1 a massive1 dark violet-blue diopside from ,san ,marcel1 ,piedmont1 ,italy1 also makes a handsome polished stone4 ,rhodonite1 silicate of manganese1 ,mn,si,o#c1 possesses a fine red colour1 and makes an attractive stone when cut and polished4 ,it has very slight biaxial double refraction1 the refractivity being about #agc2 the specific gravity is #cf1 and hardness #f4 ,it is found in large masses near ,ekaterinburg in the ,ural ,mountains1 and is quarried as an ornamental stone4 ,both the copper carbonates1 azurite or chessylite1 and malachite1 make effective polished stones4 ,the latter is also worked into various ornamental objects2 it occurs in fibrous masses1 the grained character of which look well in the polished section4 ,its colour is a bright green1 to which it owes its name1 from 1 mallows4 ,its composition is represented by the formula ,cu,c,o#c4;,cu7,,oh7#b1 and it is the more stable form1 since azurite is frequently found altered to it4 ,it has biaxial double refraction1 and the indices are about #ahh2 the specific gravity is #dja1 and hardness about #c to #d on ,mohs's scale4 ,it is found in large masses at the copper mines of ,nizhni ,tagilsk in the ,ural ,mountains1 where it is mined as an ornamental stone2 it also accompanies the copper ores in many parts of the world1 for instance ,cuba1 ,chili1 and ,australia4 ,azurite1 so called on account of its beautiful blue colour1 is rarer1 but1 unlike malachite1 is generally in the form of crystals4 ,beautiful specimens have come from ,chessy1 near ,lyons1 ,france1 and ,bisbee1 ,arizona1 ,u4,s4,a4 ,the composition corresponds to the formula #b;,cu,c,o#c1 ,cu7,,oh7#b4 ,the specific gravity is #chj1 and hardness about #c to #d4 ,chrysocolla occurs in blue and bluish-green earthy masses1 with an enamel-like texture1 which in some instances can be worked and polished4 ,being the result of the decomposition of copper ores1 it varies considerably in hardness1 ranging from #b to #d on ,mohs's scale4 ,its composition approaches to the formula ,cu,si,o#c4b;,h#b,o1 but it invariably contains impurities4 ,it is very light1 the density being only about #bb4 ,steatite1 or soapstone1 is a massive foliated silicate of magnesium corresponding to the formula ,h#b,mg#c,si#d,o#ab1 which is one of the softest of mineral substances1 representing the degree #a on ,mohs's scale1 but in massive pieces is harder owing to the intermixture of other substances with it4 ,it has a peculiar greasy feeling to the touch1 due to its softness4 ,the specific gravity is about #bge4 ,the ,chinese carve images out of the yellowish and brownish pieces4 ,meerschaum1 a silicate of magnesium corresponding to the formula ,h#d,mg#b,si#c,o#aj1 is familiar to every smoker as a material for pipe-bowls4 ,it is very light1 the specific gravity being only #bj1 and soft1 the hardness being about #b to #b on ,mohs's scale4 ,when found1 it is pure white in colour1 and answers to its name1 a ,german word signifying sea-foam4 ,it comes from ,asia ,minor4 ,serpentine has been largely used for decorative purposes1 as well as for cameos and intaglios1 and formed most of the famous #8verde antique4' ,being the result of the decomposition of other silicates it varies enormously in appearance and characters1 but the most attractive stones are a rich oil-green in colour and resemble jade4 ,the composition approximates to the formula ,h#d,mg#c,si#b,o#i1 but it invariably contains other elements4 ,the hardness varies from #b to #d on ,mohs's scale1 according to the minerals contained in the stone2 the specific gravity is about #bfj and the refractivity #aegj4 ,the beautiful rose-red stone1 thulite1 makes a handsome decorative stone4 ,it has nearly the same composition as epidote 7p4 #bge71 and like it has strong dichroism1 the principal colours being yellow1 light rose1 and deep rose4 ,the colour is due to manganese4 ,its refractive index is about #agj1 specific gravity #cab1 and hardness #f to #f on ,mohs's scale2 it possesses an easy cleavage4 ,fine specimens come from ,telemark1 ,norway1 and it is therefore called after the old name for ,norway1 ,thule4 ,marble is a massive calcite1 carbonate of lime1 with the formula ,ca,c,o#c4 ,when pure it is white1 but it is usually streaked with other substances which impart a pleasing variety to its appearance4 ,it is always readily recognized by the immediate effervescence set up when touched with a drop of acid4 ,calcite is highly doubly refractive 7cf4 p4 #dj71 the extraordinary index being #adhf1 and ordinary #afeh1 a difference of #jagb2 the specific gravity is #bga1 and hardness #c on ,mohs's scale4 ,lumachelle1 or fire-marble1 is a limestone containing shells from which a brilliant1 fire-like chatoyancy is emitted when light is reflected at the proper angle4 ,it sometimes resembles opal-matrix1 but is easily distinguished by its lower hardness and by its effervescent action with acid4 ,choice specimens come from ,bleiberg in ,carinthia1 and from ,astrakhan4 ,apophyllite has not many characters to commend it1 being at the best faintly pinkish in colour1 and always imperfectly transparent4 ,it is a hydrous silicate of potassium and calcium with the complex formula 7,h1,k7#b;,ca7,si,o#c7b4;,h#b,o4 ,its refractivity is about #aece1 specific gravity #be1 and hardness #d on ,mohs's scale2 it possesses an easy cleavage4 ,it occurs in the form of tetragonal crystals at ,andreasberg in the ,harz ,mountains1 and in the ,syhadree ,mountains1 ,bombay1 ,india4 ,,part ,,ii--,,section ,d ,,organic ,,products ,,chapter ,,xl ,,pearl1 ,,coral1 ,,amber ,although none of the substances considered in this chapter come within the strict definition of a stone1 since they are directly the result of living agency1 yet pearl at least cannot be denied the title of a gem4 ,both pearl and coral contain calcium carbonate in one or other of its crystallized forms1 and both are gathered from the sea2 but otherwise they have nothing in common4 ,amber is of vegetable origin1 and is a very different substance4 ,,pearl ,from that unrecorded day when some scantily clothed savage seeking for succulent food opened an oyster and found to his astonishment within its shell a delicate silvery pellet that shimmered in the light of a tropical sun1 down to the present day1 without intermission1 pearl has held a place all its own in the rank of jewels4 ,though it be lacking in durability1 its beauty cannot be disputed1 and large examples1 perfect in form and lustre1 are sufficiently rare to tax the deepest purse4 ,the substance composing the pearl is identical with the iridescent lining--mother-o'-pearl or nacre1 as it is termed--of the shell4 ,tortured by the intrusion of some living thing1 a boring parasite1 a worm1 or a small fish1 or of a grain of sand or other inorganic substance1 and without means to free itself1 the mollusc perforce neutralizes the irritant matter by converting it into an object of beauty that eventually finds its way into some jewellery cabinet4 ,built up in a haphazard manner and not confined by the inexorable laws of intermolecular action1 a pearl may assume any and every variety of shape from the regular to the fantastic4 ,it may be truly spherical1 egg- or pear-shaped--pear-drops or pear-eyes1 as they are termed--or it may be quite irregular--the so-called baroque or barrok pearls4 ,the first is the most prized1 but a well-shaped drop-pearl is in great demand for pendants or ear-rings4 ,the colour is ordinarily white1 or faintly tinged yellowish or bluish1 and somewhat rarely1 salmon-pink1 reddish1 or blackish grey4 ,perfect black pearls are valuable1 but not as costly as the finest of the white4 ,though not transparent1 pearl is to a varying extent translucent1 and its characteristic lustre--#8orient' in the language of jewellery--is due to the same kind of interaction of light reflected from different layers that has been remarked upon in the case of opal and certain other stones4 ,the translucency varies in degree1 and some jewellers speak of the #8water' of pearls just as in the case of diamonds4 ,if a pearl be sliced across the middle and the section be examined under the microscope1 it will be seen that the structure consists of concentric shells and resembles that of an onion4 ,these shells are alternately composed of calcium carbonate in its crystallized form1 aragonite1 and of a horny organic matter known as conchiolin1 and they evidently represent the result of intermittent growth4 ,because of their composite character1 pearls have a specific gravity ranging from #bfe to #bfi-bhd-bhi in the case of pink pearls--which is appreciably less than that of aragonite1 #bid3 the hardness is about the same1 namely1 #c to #d on ,mohs's scale4 ,that the arrangement of the mineral layers is approximately parallel is evinced by the distinctness of the shadow-edges shown on examination with the refractometer4 ,pearls require very careful handling1 both because they are comparatively soft and therefore apt to be scratched1 and because they are chemically affected by acids1 and even by the perspiration from the skin4 ,acids attack only the calcium carbonate1 not the organic matter2 the well-known story therefore of ,cleopatra dissolving a valuable pearl in vinegar1 which is moreover1 too weak an acid to effect the solution quickly1 must not be accepted too literally4 ,pearls are not cut like stones1 and therefore as soon as the precious bloom has once gone1 nothing can be done to revive it4 ,attempts are sometimes made in the case of valuable pearls to remove the dull skin and lay bare another iridescent layer underneath1 but the operation is exceedingly delicate4 ,even with the best of care pearls must in process of time perish owing to the decay of the organic constituent4 ,pearls that have been discovered in ancient tombs crumbled to dust at a touch1 and those formerly in ancient rings have vanished or only remain as a brown powder1 while the garnets or other stones set with them are little the worse for the centuries that have passed by4 ,the largest known pearl was at one time in the famous collection belonging to the banker1 ,henry ,philip ,hope4 ,cylindrical in form1 with a slight swelling at one end1 it measures #ej mm4 7#b inches7 in length1 and #aae mm4 7#d inches7 in circumference about the thicker1 and #hc mm4 7#c inches7 about the thinner end1 and weighs #ded carats4 ,about three-quarters of it is white in colour with a fine #8orient1' and the remainder is bronze in tint4 ,it is valued at upwards of #ab1jjj4 ,a large pearl1 #cjj carats in weight1 is in the imperial crown of the ,emperor of ,austria1 and another1 pear-shaped1 is in the possession of the ,shah of ,persia4 ,a beautiful white ,india pearl1 a perfect sphere in shape1 and #bh carats in weight1 is in the ,museum of ,zosima in ,moscow2 it is known as #8,la ,pellegrina4' ,the #8,great ,southern ,cross1' which consists of nine large pearls naturally joined together in the shape of a cross1 was discovered in an oyster fished up in #ahhf off the beds of ,western ,australia4 ,the collection of jewels in the famous ,green ,vaults at ,dresden contains a number of pearls of curious shapes4 ,large pearls are sold separately1 while the small pearls known as #8seed' pearls come into the market bored and strung on silk in #8bunches4' ,the unit of weight is the pearl grain1 which is a quarter of a carat1 and the rate of price depends on the square of the weight in grains4 ,the rate per unit or base varies from #f;d4 to #ejs4 according to the shape and quality of the pearl4 ,spherical pearls command the best prices1 next the pearl-drops1 and lastly the buttons2 but whatever the shape1 it is imperative that the pearl have #8orient1' without which it is valueless4 ,the cheaper grades of pearls are sold by the carat4 ,illustration3 ,,plate ,,xxx ,,natives ,,drilling ,,pearls ,for use in necklaces and pendants pearls are bored with a steel drill1 and threaded with silk1 an easy operation on account of their softness4 ,they harmonize well with diamonds4 ,small pearls are often set as a frame to large coloured stones1 to which they form an admirable foil4 ,pearls set in rings or anywhere where the upper half alone would show are generally sawn in halves2 #8button' pearls find an extensive use in modern rings4 ,any mollusc1 whether of the bi-valve or the uni-valve type1 which possesses a nacreous shell1 has the power of producing pearls1 but only two1 the pearl-oyster1 ,meleagrina margaritifera1 and the pearl-mussel1 ,unio margarifer1 repay the cost of systematic fishing4 ,the outside of the shell is formed of the horny matter called conchiolin2 while the inside is composed of two coats1 of which the outer consists of alternate layers of conchiolin and calcium carbonate in its crystallized form1 calcite1 and the inner of the same organic matter1 but with calcium carbonate in its other crystallized form1 aragonite4 ,the latter coat forms the nacreous lining known as mother-o'-pearl1 which is identical in consistency with pearl1 but somewhat more transparent4 ,the iridescence of mother-o'-pearl is due not only to the fact that it is composed of a succession of thin translucent layers1 but also to the fact that these layers overlap like slates on a house1 and form a series of fine parallel lines on the surface2 diffraction therefore as well as interference of light takes place1 and a similar diffraction phenomenon is displayed even by a cast of the inside of the shell4 ,the animal has the property of secreting calcium carbonate1 which it absorbs from the sea-water1 in both its crystallized conditions as well as conchiolin4 ,at the outer rim it secretes conchiolin1 further in calcite1 and at the very inside aragonite4 ,the shape and appearance of a pearl therefore depend on the position in which the intruding substance is situated within the shell4 ,the most perfect pearl has been in intermittent motion in the interior of the mollusc1 and has received successive coats according to the position in which it happened to be4 ,a parasite that bores into the shell is walled up at the point of entrance1 and a wart- or blister-pearl results4 ,the thinner the successive coats the finer the lustre4 ,pearls have even been discovered embedded in the animal itself4 ,the number of pearls found in a shell depends on the number of times the living host was compelled to seal up some irritant object1 and may vary from one up to the eighty-seven which are said to have been found in an ,indian oyster4 ,that an oyster thus distinguished has not led a happy existence is testified by the distorted shape of its shell1 a clue that guides the pearl-fishers in their search4 ,moreover1 pearl-oysters never have thick nacreous shells1 and on the other hand molluscs with fine mother-o'-pearl seldom contain pearls4 ,beautiful white and silvery pearls are found in a small oyster that lives at a depth of #f to #ac fathoms 7#aa-bd m47 in the ,gulf of ,manaar1 off the coast of ,ceylon4 ,about seven-eighths1 however1 of the pearls that come into the market are obtained from a larger oyster which has its home on the ,arabian coast of the ,persian ,gulf4 ,these famous fisheries have been known since very early times4 ,the pearls found here are more yellowish than those from ,ceylon1 but are nevertheless of excellent quality4 ,the pearl fisheries off the north-west coast of ,western ,australia and off ,venezuela are also not unimportant1 and fine black pearls have been supplied by molluscs from the ,gulf of ,mexico4 ,illustration3 ,,plate ,,xxxi ,,metal ,,figures ,,of ,,buddha ,,inserted ,,in ,a ,,pearl-,,oyster ,illustration3 ,,plate ,,xxxii ,,fig4 #a ,,fig4 #b ,,sections ,,of ,,culture ,,pearl ,,fig4 #a4 ,,in ,,the ,,oyster4 ,,fig4 #b4 ,,when ,,finished4 ,a4 ,,pearly ,,deposit4 ,b4 ,,piece ,,of ,,mother-,o'-,,pearl ,,inserted ,,in ,,the ,,oyster4 ,c4 ,,outer ,,shell ,,of ,,the ,,oyster4 ,d4 ,,mother-,o'-,,pearl ,,back ,,added4 ,the ,chinese have long made a practice of introducing into the shell of a pearl-oyster little tin images of ,buddha in order that they may be coated with the nacreous secretion4 ,the ,japanese have during recent years made quite an industry of stimulating the efforts of the mollusc by cementing small pieces of mother-o'-pearl to the interior surface of the shell 7,plate ,,xxxii1 ,fig4 #a72 these #8culture' pearls1 as they are termed1 are recognizable by examination of the back4 ,about a year has to elapse before a coating of a tenth of a millimetre is formed1 and another two years must pass before the thickness is doubled4 ,after removal the piece of mother-o'-pearl1 which is now coated with several nacreous layers1 is cemented to a piece of ordinary mother-o'-pearl1 and the lower portion is ground to the usual symmetrical shape 7,plate ,,xxxii1 ,fig4 #b74 ,blister pearls are often similarly treated4 ,in both cases1 however1 the #8orient' is deficient in quality4 ,the finest mother-o'-pearl is supplied by a mollusc found in the sea near the islands lying between ,borneo and the ,philippines1 and fine material is found at ,shark ,bay and off ,thursday ,island4 ,,coral ,coral ranks far below pearl and meets with but limited appreciation4 ,it is common enough in warm seas1 but the only kind which finds its way into jewellery is the rose or red-coloured coral--the noble coral1 ,corallium nobile or rubrum4 ,it consists of the axial skeleton of the coral polyp1 and is built up of hollow tubes fitting one within the other4 ,the composition is mainly calcium carbonate with a little magnesium carbonate and a small amount of organic matter4 ,the former of the mineral substances is in the form of calcite1 and the crystals are arranged in fibrous form radiating at right angles to the axis of the coral4 ,the specific gravity varies from #bf to #bg1 being slightly under that of calcite1 and the hardness is somewhat greater1 being about #c on ,mohs's scale4 ,the best red coral is found in the ,mediterranean ,sea off ,algiers and ,tunis in ,africa1 and ,sicily and the ,calabrian ,coast of ,italy4 ,the industry of shaping and fashioning the coral is carried on almost entirely in ,italy4 ,coral is usually cut into beads1 either round or egg-shaped1 and used for necklaces1 rosaries1 and bracelets4 ,the best quality fetches from #bjs4 to #cjs4 per carat4 ,,amber ,this fossil resin1 yellow and brownish-yellow in tint1 finds an extensive use as the material for mouthpieces of pipes1 cigar and cigarette-holders1 umbrella-handles1 and so on1 and is even locally cut for jewellery1 although its extreme softness1 its hardness being only #b on ,mohs's scale1 quite unfits it for such a purpose4 ,it is only slightly denser than water1 the specific gravity being about #aaj4 ,since the structure is amorphous the refraction is single1 the index being about #aedj4 ,amber1 being a very bad conductor of heat1 is perceptibly warm to the touch4 ,its property of becoming electrified by friction attracted early attention1 and from the ,greek name for it1 1 is derived our word electricity4 ,amber is washed up by the sea off the coasts of ,sicily and ,prussia1 and of ,norfolk and ,suffolk in ,england4 ,the finest examples1 which are picked up off the shore of ,catania in ,sicily1 are distinguished by a fine bluish fluorescence1 resembling that seen in lubricating oil2 such pieces command good prices4 ,a recent resin1 pale yellow in colour1 known as kauri-gum1 is found in ,new ,zealand1 where it is highly valued4 ,,tables ,,table ,i ,chemical ,composition of ,gem-,stones 7a7 ,,elements-- ,diamond ,c 7b7 ,,oxides-- ,corundum ,al#b,o#c ,quartz ,si,o#b ,chalcedony ,si,o#b ,opal ,si,o#b4n,h#b,o 7c7 ,,aluminates-- ,spinel ,mg,al#b,o#d ,chrysoberyl ,be,al#b,o#d 7d7 ,,silicates-- ,phenakite ,be#b,si,o#d ,dioptase ,h#b;,cu,si,o#d ,peridot ,mg#b,si,o#d ,zircon ,zr,si,o#d ,enstatite ,mg,si,o#c ,diopside ,ca,mg7,si,o#c7b ,nephrite ,ca,mg#c7,si,o#c7d ,sphene ,ca,ti,si,o#e ,benitoite ,ba,ti,si#c,o#i ,andalusite ,al7,al,o7,si,o#d ,kyanite 7,al,o7#b,si,o#c ,topaz ,al7,f1,,oh7#b,si,o#d ,epidote ,ca#b7;,al1,fe7#b7;,al,o,h77,si,o#d7c ,euclase ,be7,al,o,h7,si,o#d ,prehnite ,h#b;,ca#b;,al#b7,si,o#d7c ,iolite ,h#b7,mg1,fe7#d;,al#h,si#aj,o#cg ,hessonite ,ca#c;,al#b7,si,o#d7c ,garnet ,pyrope ,mg#c;,al#b7,si,o#d7c ,almandine ,fe#c;,al#b7,si,o#d7c ,andradite ,ca#c;,fe#b7,si,o#d7c ,beryl ,be#c;,al#b7,si,o#c7f ,spodumene ,li,al7,si,o#c7b ,jadeite ,na,al7,si,o#c7b ,moonstone ,k,al,si#c,o#h ,tourmaline#ab,si,o#b4c;,b#b,o#c47i-x77, al1,fe7#b,o#c4cx 7,fe1,mn1,ca1,mg1,k#b1,na#b1,li#b1;,h#b7 ,o4#c;,h#b,o ,axinite ,h,ca#c;,al#b;,b7,si,o#d7d ,idocrase 7,ca1,mn1,mg1,fe7#b7;,al1,fe77,,oh1,f7,s i#b,o#g 7e7 ,,phosphates-- ,beryllonite ,na,be,p,o#d ,apatite ,ca#e7;,f1,cl77,,po#d7c ,turquoise ,cu,o,h4#f;,al7,,oh7#b4;,h#e47,,po#d7d ,,table ,,ii ,colour of ,gem-,stones ,colourless and ,white4--,diamond1 corundum 7white sapphire71 topaz1 quartz 7rock-crystal71 zircon 7when #8fired'71 moonstone2 rarely beryl1 tourmaline2 among the less common species1 phenakite1 spodumene 7colourless kunzite71 beryllonite4 ,yellow4--,diamond1 topaz1 corundum 7yellow sapphire71 quartz 7citrine1 ,scotch or occidental topaz71 tourmaline1 zircon1 sphene1 spodumene1 beryl4 ,pink and ,lilac4--,corundum 7pink sapphire71 spinel 7balas-ruby71 tourmaline 7rubellite71 topaz 7usually when #8fired'71 spodumene 7kunzite71 beryl 7morganite71 quartz 7rose-quartz74 ,red4--,corundum 7ruby71 garnet 7pyrope1 almandine71 spinel 7balas-ruby71 tourmaline 7rubellite71 zircon1 opal 7fire-opal74 ,green4--,beryl 7emerald1 aquamarine71 peridot1 corundum1 tourmaline1 chrysoberyl 7including alexandrite71 zircon1 garnet 7demantoid72 among less common species1 spodumene 7hiddenite71 euclase1 diopside1 idocrase1 epidote1 apatite1 obsidian2 rarely diamond2 also semi-opaque1 turquoise1 jade4 ,blue4--,corundum 7sapphire71 spinel1 topaz1 tourmaline1 zircon2 among the less common species1 kyanite1 iolite1 benitoite1 apatite2 rarely diamond2 also semi-opaque1 turquoise1 lapis lazuli1 sodalite4 ,violet and ,purple4--,quartz 7amethyst71 corundum 7oriental amethyst71 spinel 7almandine-spinel71 garnet 7almandine71 spodumene 7kunzite71 apatite4 ,brown4--,diamond1 tourmaline1 quartz 7smoky-quartz72 among the less common species1 andalusite1 axinite1 sphene4 ,,table ,,iii ,refractive ,indices of ,gem-,stones#h ,opal #aded ,moonstone #aec #aed ,iolite #aedc #aeea ,quartz #aedd #aeec ,beryllonite #aeec #aefe ,beryl #aegh #aehe ,turquoise #afa #afe ,topaz #afah #afbg ,andalusite #afcb #afdc ,tourmaline #afbf #afea ,apatite #afdb #afdf ,phenakite #afeb #affg ,euclase #afea #afgj ,spodumene #affj #afge ,enstatite #affe #afgd ,peridot #afei #afig ,axinite #afgd #afhd ,diopside #afhe #agje ,idocrase #agad #agai ,spinel #agbf ,kyanite #agb #agc ,epidote #agce #agff ,garnet 7,hessonite7 #agde ,chrysoberyl #agdf #agec ,garnet 7,pyrope7 #agee ,benitoite #ageg #ahjd ,corundum #agfa #aggj ,garnet 7,almandine7 #agij ,zircon 7a7 #ahae ,garnet 7,demantoid7 #ahhe ,sphene #aija #aihe ,zircon 7b7 #aibg #aihj ,diamond #bdag ,,table ,,iv ,colour-,dispersion of ,gem-,stones#i ,moonstone #jab ,quartz #jac ,beryl #jad ,topaz #jad ,chrysoberyl #jae ,tourmaline #jag ,spodumene #jag ,corundum #jah ,peridot #jbj ,spinel #jbj ,garnet 7,almandine7 #jbd ,garnet 7,pyrope7 #jbg ,garnet 7,hessonite7 #jbh ,zircon #jch ,diamond #jdd ,sphene #jea ,garnet 7,demantoid7 #jeg ,,table ,v ,character of the ,refraction of ,gem-,stones 7a7 ,,single-- ,diamond1 spinel1 garnet1 opal4 ,diamond and garnet frequently display local double refraction4 7b7 ,,uniaxial1 ,,positive-- ,quartz #jji ,phenakite #jae ,benitoite #jdg ,zircon 7b7 #jec ,quartz exhibits circular polarization4 7c7 ,,uniaxial1 ,,negative-- ,apatite #jjd ,idocrase #jje ,beryl #jjg ,corundum #jji ,tourmaline #jbe 7d7 ,,biaxial1 ,,positive-- ,chrysoberyl #jjg ,topaz #jji ,enstatite #jji ,spodumene #jae ,euclase #jai ,diopside #jbj ,peridot #jch ,sphene #jhd 7e7 ,,biaxial1 ,,negative-- ,moonstone #jjf ,iolite #jjh ,axinite #jaj ,andalusite #jaa ,beryllonite #jab ,kyanite #jaf ,epidote #jca ,,table ,,vi ,dichroism of ,gem-,stones 7a7 ,,strong ,corundum1 tourmaline1 alexandrite1 spodumene1 andalusite1 iolite1 epidote1 axinite4 7b7 ,,distinct ,emerald1 topaz1 quartz1 peridot1 chrysoberyl1 enstatite1 euclase1 idocrase1 kyanite1 sphene1 apatite4 7c7 ,,weak ,beryl1 diopside4 ,,table ,,vii ,specific ,gravities of ,gem-,stones ,opal #bae ,moonstone #beg ,iolite #bfc ,quartz #bff ,beryl #bgd ,turquoise #bhb ,beryllonite #bhd ,phenakite #bii ,euclase #cjg ,tourmaline #caj ,enstatite #caj ,andalusite #cah ,spodumene #cah ,apatite #cbj ,axinite #cbh ,diopside #cbi ,epidote #ccg ,peridot #cdj ,idocrase #cdj ,sphene #cdj ,diamond #ceb ,topaz #cec ,spinel #cfj ,kyanite #cfa ,garnet 7,hessonite7 #cfa ,benitoite #cfd ,chrysoberyl #cgc ,garnet 7,pyrope7 #cgh ,garnet 7,demantoid7 #chd ,corundum #djc ,garnet 7,almandine7 #dje ,zircon 7a7 #dbj ,zircon 7b7 #dfi ,,table ,,viii ,degrees of ,hardness of ,gem-,stones #e4 ,kyanite 7#e-g71 apatite1 lapis lazuli #e4 ,enstatite1 beryllonite1 sphene #f4 ,opal1 moonstone1 turquoise1 diopside #f4 ,spodumene1 peridot1 garnet 7demantoid71 benitoite1 idocrase1 epidote1 axinite1 jade 7nephrite7 #g4 ,iolite1 quartz1 tourmaline1 jade 7jadeite7 #g4 ,garnet 7hessonite1 pyrope7 #g4 ,beryl1 garnet 7almandine71 zircon1 phenakite1 euclase1 andalusite #h4 ,topaz1 spinel #h4 ,chrysoberyl #i4 ,corundum #aj4 ,diamond ,,table ,,ix4--,,data ,densities of ,water and ,toluol at ,ordinary ,temperatures ---------------------------------------- --------- ,,temperature ,,water ,,toluol ---------------------------------------- --------- ,centigrade ,fahrenheit #ad #egb #jiiid #jhfig #ae #eij #jiiib #jhfhg #af #fjh #jiiij #jhfgg #ag #fbf #jiihh #jhffg #ah #fdd #jiihf #jhfeg #ai #ffb #jiihe #jhfdg #bj #fhj #jiihc #jhfcg #ba #fij #jiiha #jhfbg #bb #gaf #jiigi #jhfag #bc #gcd #jiigg #jhfjg ---------------------------------------- --------- #a ,english carat #jbjec gram #a ,metric carat #jbjjj 7one-fifth7 gram #a oz4 ,av4 #bhce grams #a lb4 ,av4 #jdecf kilogram #a inch #bed millimetres #a foot #jcjdh metre #a yard #jiadd metre #a mile #afjic kilometre ,,index ,absorption1 #ec1 #ei ,absorption spectra1 #ei ,achroite1 #bbj1 #bba ,adularia1 #bee ,agate1 #bdg ,akbar ,shah diamond1 #afc ,alalite1 #bgb ,albite1 #bed ,alexandrite1 #ed1 #fj1 #bcc ,scientific1 #abb ,almandine1 #fj1 #bad ,oriental1 #aab1 #agb spinel1 #aab1 #bjd ,amazon-stone1 #bee ,amber1 #hc1 #bih ,amethyst1 #bci1 #bdb ,oriental1 #aaa1 #agb1 #bci ,anatase1 #bha ,andalusite1 #bgd ,andradite1 #baf ,anomalous refraction1 #dg ,anorthite1 #bed ,apatite1 #bgi ,apophyllite1 #bij ,aquamarine1 #ahd1 #aic ,arizona-ruby1 #bac ,artificial stones1 #abd ,asteria1 #ch1 #agg ,asterism1 #ch ,australia stones1 #aed1 #agd1 #ahb1 #aie1 #bac1 #baf1 #bbg1 #bcb1 #beb1 #bhh ,austrian ,yellow diamond1 #afe ,aventurine1 #bdj1 #bda ,axes1 ,crystallographic1 #i ,optic1 #di ,axinite1 #bgh ,azure-quartz1 #bdd ,azurite1 #bhg ,balas-ruby1 #bjc ,barnato1 ,barnett1 #ade ,baroque1 ,barrok1 pearls1 #bib ,bastite1 #bgb ,benitoite1 #bfg ,berquem1 ,louis de1 #ij1 #afa ,beryl1 #ahd ,beryllonite1 #bgj ,bezel facet1 #ib ,biaxial double refraction1 #de1 #di1 #eg ,bisectrix1 #de1 #di ,black diamond1 #abi ,black lead1 #abi ,black opal1 #bdi1 #bej ,black ,prince's ruby1 #bjf ,blister-pearl1 #bif ,bloodstone1 #bdg ,blue felspar1 #bee ,blue ground1 #adc1 #adg ,blue ,john1 #bhe ,boart1 #ajc1 #abi1 #acc ,bohemian garnet 7pyrope71 #bjg1 #bab ,bone turquoise1 #bei ,boodt1 ,a4 ,b4 de1 #acb1 #bac ,borgis1 ,hortensio1 #afa ,borneo stones1 #aed1 #agj ,bort1 v4 ,boart1 #ajc1 #abi1 #acc ,bottle-stone1 #bhd ,boule1 #aah ,bowenite1 #bfc ,braganza diamond1 #agj ,brazil stones1 #ach1 #afe1 #aff1 #afi1 #aid et seq41 #bja1 #bae1 #bbc1 #bcf1 #bdc1 #bdd1 #bdh1 #bff1 #bfi1 #bgj1 #bgd ,brazilian emerald1 #aaa1 #bbj1 #bba peridot1 #bba sapphire1 #aaa1 #bba topaz1 #aaa1 #aig ,brilliant form of cutting1 #ib ,brilliant1 ,scientific1 #abb ,bristol diamonds1 #bdc ,bruting1 #ajj ,burma stones1 #agh1 #bje1 #bbc1 #bbg1 #bfc ,button-pearl1 #bie ,byes1 ,bywaters1 #acf1 #aej ,cabochon form of cutting1 #hh ,cacholong1 #bea ,cairngorm1 #bci ,callaica1 callaina1 callais1 #beh ,calcite1 #dj1 #bhi ,california stones1 #aef1 #aie1 #bjb1 #bbd1 #bei1 #bfe1 #bfg1 #bge ,californite1 #bfd1 #bge ,cape-ruby1 #bac ,carat weight1 #gb1 #hd ,carbon1 #abi ,carbonado1 #abi ,carborundum1 #aje ,carbuncle1 #hi1 #bae ,carnelian1 #bdg ,cascalho1 #aci ,cassiterite1 #bha ,cat's-eye 7chrysoberyl71 #ch1 #ij1 #bcc 7quartz71 #ci1 #ij1 #bdj 7tourmaline71 #ci1 #bai ,hungarian1 #bdd ,ceylon stones1 #aha1 #aie1 #bja1 #bje1 #bab1 #bae1 #baf1 #bbc1 #bcb1 #bcf1 #bcg1 #bdc1 #bdd1 #bee1 #bfg1 #bgd1 #bgi1 #bhd ,ceylonese peridot 7tourmaline71 #bba ,ceylonite1 #bjd ,chalcedony1 #bdf ,chatoyancy1 #ch ,chert1 #bdg ,chessylite1 #bhg ,chrysoberyl1 #bcc ,chrysocolla1 #bhh ,chrysolite 7chrysoberyl71 #bcc 7peridot71 #bbe ,chrysoprase1 #bdg ,church1 ,sir ,arthur1 #fa1 #baa1 #bca ,cinnamon-stone1 #baa ,citrine1 #bci ,cleavage1 #hj1 #ajj1 #adi ,close goods1 #adi ,colenso diamond1 #aca ,colour1 #ec ,colour dispersion1 #bj1 #ig ,conchiolin1 #bic ,coral1 #bih ,cordierite1 #bff ,cornish diamonds1 #bdc ,corundum1 #agb ,crocidolite1 #ci1 #bdj ,crookes1 ,sir ,william1 #acb1 #aec ,cross facet1 #ic ,crystal1 #f1 #g1 #h ,rock-1 #ig ,cubic system1 #h ,culet facet1 #ic ,cullinan diamond1 #id1 #ajj1 #afh ,culture pearls1 #big ,cumberland diamond1 #afd ,cyanite 7,kyanite71 #gi1 #bgc ,cymophane1 #bcd ,darya-i-nor diamond1 #afb ,de ,beers diamonds1 #afg ,demantoid1 #baf ,density1 #fc ,deviation1 ,minimum1 #cj ,diamond1 ,characters of1 #abh cutting1 #ij gauges1 #hf ,glaziers'1 #ace mining1 #adf ,occurrence of1 in-- ,borneo1 #aed ,brazil1 #aci ,german ,south-,west ,africa1 #aee ,india1 #ach ,new ,south ,wales1 #aed ,rhodesia1 #aee ,south ,africa1 #aci ,origin of1 #aea -point1 #ia -rose1 #ib -table1 #ia ,diamonds1 ,classification of1 #acf1 #adi ,historical1 #aeg ,prices of1 #ace ,dichroism1 #ee ,dichroite1 #bff ,dichroscope1 #ee ,diffusion column1 #fe ,diopside1 #bgb ,dioptase1 #bhj ,dispersion1 ,colour1 #bj1 #bd1 #ig ,disthene1 #bgc ,dop1 #ajb ,double refraction1 #bh1 #dj ,doublet1 #abe ,dresden diamond1 #aga ,drop-stone1 #id ,duke of ,devonshire's emerald1 #aia ,edwardes ruby1 #age ,electrical characters1 #hb ,emerald1 #hi1 #ahd ,brazilian1 #bbj1 #bba ,evening1 #bbe ,oriental1 #aaa1 #agb ,scientific1 #abb ,uralian1 #baf ,emeraldine1 #bdg ,emery1 #age ,english ,dresden diamond1 #aff ,enstatite1 #bga ,epidote1 #bge ,essence d',orient1 #abf ,essonite 7,hessonite71 #baa ,euclase1 #bfi ,eugnie diamond1 #afd ,evening emerald1 #bbe ,excelsior diamond1 #afg ,extinction1 #de ,faceting machine1 #aje ,false topaz1 #bci ,felspar1 #bed ,fire1 #bj1 #if ,fire-marble1 #bhi ,fire-opal1 #bea ,flats1 #aej ,flches d'amour1 #bdj ,flint1 #bdg ,floors1 #adg ,fluor1 #bhe ,frmy1 ,e41 #aae ,garnet1 #bjg ,green1 #bga ,gaudin1 ,m4 ,a4 ,a41 #aae ,gauges1 ,diamond1 #hf ,girdle1 #ib ,glass1 #g1 #abd ,gnaga ,boh ruby1 #ahj ,goniometer1 #cj ,grain1 ,pearl1 #hf ,graphite1 #abi ,greaser1 #adi ,great ,mogul diamond1 #afa ,great ,southern ,cross group of pearls1 #bid ,great ,table diamond1 #afb ,great ,white diamond1 #afg ,green garnet1 #bga ,greenstone1 #bfa ,grossular1 #baa ,habit1 #ab ,hardness1 #gh ,haynite1 #bhf ,heavy liquids1 #fd ,hematite1 #bhb ,hessonite1 #baa ,hexagonal system1 #aj ,hiddenite1 #bff ,hope cat's-eye1 #bcg chrysolite1 #bcg diamond1 #agj pearl1 #bid sapphire1 #aba ,hornstone1 #bdg ,hungarian cat's-eye1 #bdd ,hyacinth1 #baa1 #bbh ,hydrophane1 #bej ,hydrostatic weighing1 #gb ,hypersthene1 #bga ,iceland-spar1 #dj1 #dd ,idocrase1 #bgd ,imitation stones1 #abd ,imperial diamond1 #afg ,index of refraction1 #af ,india stones1 #acg1 #aha1 #aid1 #bae1 #bdc1 #bdd1 #bdh1 #bij ,indicators1 #fe ,indicolite1 #bba ,interference of light1 #ci1 #dh ,iolite1 #bff ,iris1 #bdj ,isle of ,wight diamonds1 #bdc ,isomorphous replacement1 #ac1 #ai ,jacinth1 #baa1 #bbh ,jade1 #bfj ,jadeite1 #bfb ,jargoon1 #bbh ,jasper1 #bdg ,jehan ,ghir ,shah diamond1 #afc ,jigger1 #adi ,jubilee diamond1 #afg ,kauri-gum1 #bii ,khiraj-i-,alam ruby1 #bjf ,kimberlite1 #aeb ,king topaz1 #aha1 #bja ,klein's solution1 #fg ,koh-i-nor diamond1 #acg1 #aeh ,kunz1 ,dr4 ,g4 ,f41 #ahf1 #bbd1 #bfb1 #bfe ,kunzite1 #bfe ,kyanite1 #gi1 #bgc ,labradorite1 #bee ,la ,pellegrina pearl1 #bid ,lapis lazuli1 #bhf ,lazurite1 #bhf ,lozenge facet1 #ic ,lumachelle1 #bhi ,lustre1 #cg ,maacles1 ,macles1 #ab1 #aej ,madagascar stones1 #aie1 #bbd1 #bdc1 #bfe1 #bff ,malachite1 #bhg ,malacolite1 #bgb ,manufactured stones1 #aac ,marble1 #bhi ,mattan diamond1 #aee1 #agj ,matura diamonds1 #bcb ,mazarin1 ,cardinal1 #ib ,meerschaum1 #bhh ,mle1 #acf ,methylene iodide1 #bf1 #ff ,metric carat1 #he1 #hg ,milky-quartz1 #bdj ,minimum deviation1 #cj ,mocha-stone1 #bdg ,moe's gauge1 #hg ,mohs's scale of hardness1 #gh ,moissan1 ,henri1 #aec ,moldavite1 #bhc ,monoclinic system1 #aa ,moon of the ,mountains diamond1 #afb ,moonstone1 #ci1 #bee ,morganite1 #ahf1 #aie ,moroxite1 #bgi ,moss-agate1 #bdg ,mother-of-emerald1 #bdj ,mother-o'-pearl1 #bib ,nacre1 #bib ,napoleon diamond1 #afd ,nassak diamond1 #afc ,negative double refraction1 #de ,nephrite1 #bfa ,nicol's prism1 #dd ,nizam diamond1 #afb ,obsidian1 #bhc ,occidental topaz1 #aaa1 #bci ,odontolite1 #bei ,off-coloured diamonds1 #acj ,olivine 7demantoid71 #baf 7peridot71 #bbe ,onyx1 #bdg ,opal1 #ci1 #bdi ,fire1 #bea -matrix1 #bea ,opalescence1 #ci ,optical anomalies1 #dg ,optic axes1 #di ,oriental almandine1 #aab1 #agb amethyst1 #aaa1 #agb emerald1 #aaa1 #agb topaz1 #aaa1 #agb ,orient of pearls1 #bib ,orloff diamond1 #afj ,orthoclase1 #bed ,orthorhombic system1 #aa ,pacha of ,egypt diamond1 #afe ,paste1 #dg1 #abd ,paul ,i diamond1 #aga ,pavilion1 #ic ,pavilion facet1 #ic ,pear-drop pearls1 #bib ,pear-eye pearls1 #bib ,pearl1 #bia grain1 #hf imitations1 #abf ,pendeloque1 #id ,peridot1 #bbe ,brazilian1 #bba ,ceylonese1 #bba ,peruzzi1 ,vincenzio1 #ib ,phenakite1 #bfi ,pigott diamond1 #afd ,pipes1 #aeb ,pistacite1 #bge ,pitt diamond1 #ajj1 #aei ,plasma1 #bdg1 #bfd ,pleochroism1 #eg ,pleonaste1 #bjd ,pliny1 #f1 #hh1 #ach1 #ahd1 #aia1 #bda1 #bdi ,polar ,star diamond1 #afc ,polarization1 #db ,porter-,rhodes diamond1 #aff ,positive double refraction1 #de ,prase1 #bdj1 #bdg ,prehnite1 #bgh ,pycnometer1 #ge ,pyrites1 #bhb ,pyrope1 #bab ,quartz1 #ej1 #bch ,quoin facet1 #ic ,rainbow-quartz1 #bdj ,reconstructed stones1 #aaf ,reef1 #add ,reflection of light1 #ad ,refraction of light1 #ae ,refractive index1 #af ,refractometer1 #bb1 #ej ,regent diamond1 #ajj1 #aei ,retgers's salt1 #fi ,rhodes1 ,cecil ,j41 #ade ,rhodesia stones1 #aee1 #ahc1 #bac1 #bcf ,rhodolite1 #fb1 #bad ,rhodonite1 #bhg ,rock-crystal1 #ig1 #bci ,rock-drill1 #acd ,rntgen rays1 #hc ,rose form of cutting1 #ia ,rose-quartz1 #bdj ,rospoli sapphire1 #ahb ,rotation of plane of polarization1 #ej ,rubellite1 #bbj1 #bbc ,rubicelle1 #bjc ,ruby1 #ih1 #aaj1 #agb ,balas-1 #bjc ,cape-1 #bac ,sancy diamond1 #afa ,sapphire1 #ih1 #aaj1 #agb ,brazilian 7tourmaline71 #bba -quartz1 #bdd ,water- 7iolite71 #bff ,water- 7topaz71 #bja ,sard1 #bdg ,sardonyx1 #bdg ,saussurite1 #bfc ,schorl1 #bba ,scientific alexandrite1 #abb brilliant1 #abb emerald1 #abb topaz1 #aba ,scotch topaz1 #bci ,seed pearls1 #bid ,serpentine1 #bhi ,setting of gem-stones1 #ajg ,shah diamond1 #afc ,sheen1 #ci ,shepherd's ,stone diamond1 #afc ,siam stones1 #ahj ,siberia and ,asiatic ,russia stones1 #ahb1 #ahh1 #aid1 #bja1 #bag1 #bbc1 #bcf1 #bdd1 #bef1 #bfb1 #bfi1 #bgj1 #bhg ,siberite1 #bba ,siderite1 #bdd ,silver-thallium nitrate1 #fi ,skew facet1 #ic ,skill facet1 #ic ,smoky quartz1 #bdj ,snell's laws1 #af ,soapstone1 #bhh ,sodalite1 #bhf1 #bhg ,sonstadt's solution1 #fg ,south ,africa stones1 #aci et seq41 #aff1 #afg et seq41 #bac1 #bcb1 #bdd1 #bfd1 #bga ,spanish topaz1 #bci ,specific gravity1 #fc ,specific-gravity bottle1 #ge ,spectroscope1 #ei ,spectrum1 #bj1 #be ,spectrum1 ,absorption1 #ei ,spessartite1 #baf ,sphene1 #bgf ,spinel1 #bjc ,spodumene1 #bfe ,spotted stones1 #adi ,star-facet1 #ib ,star of ,africa diamond1 #afh ,star of ,este diamond1 #afe ,star of ,minas diamond1 #afi ,star of ,south ,africa diamond1 #ada1 #aff ,star of the ,south diamond1 #aci1 #afe ,starstones1 #ch1 #agg ,steatite1 #bhh ,step form of cutting1 #ih ,stewart diamond1 #aff ,strass1 #abd ,sunstone1 #bee ,synthetical stones1 #aac ,syriam1 ,syrian1 garnet1 #bae ,table facet1 #ib ,table form of cutting1 #ia ,tavernier1 ,j4 ,b41 #ia1 #abi1 #acg1 #afa1 #afb1 #agj ,templet facet1 #ib ,tetragonal system1 #i ,thulite1 #bhi ,tiffany diamond1 #aga ,tiger's-eye1 #ci1 #bdj ,timur ruby1 #bjf ,titanite1 #bgf ,topaz1 #aig ,brazilian1 #aig ,false1 #bci ,occidental1 #aaa1 #bci ,oriental1 #aaa1 #agc ,scientific1 #aba ,scotch1 #bci ,spanish1 #bci ,topazolite1 #baf ,total-reflection1 #ah1 #ba ,tourmaline1 #dc1 #bai ,trap form of cutting1 #ih ,trichroism1 #eg ,triclinic system1 #ab ,triplet1 #abf ,turquoise1 #beg ,turquoise-matrix1 #beh ,tuscany diamond1 #afe ,twinning1 #ab1 #dg ,uniaxial double refraction1 #de1 #dh #eg ,uralian emerald1 #bag ,uvarovite1 #bah ,variscite1 #bei ,verdite1 #bfd ,verneuil1 ,a4 ,v4 ,l41 #aaf ,vesuvianite1 #bgd ,victoria diamond1 #afg ,violane1 #bhg ,wart-pearl1 #bif ,water 7of diamonds71 #abi 7of pearls71 #bib ,water-chrysolite1 #bhd -sapphire 7iolite71 #bff -sapphire 7topaz71 #bja ,white opal1 #bdi ,white ,saxon diamond1 #afe ,wollaston1 ,w4 ,h41 #acc ,x-rays1 #hc ,yellow ground1 #adc ,zircon1 #bbh ,printed by ,,morrison ,,gibb ,,limited1 ,edinburgh ---------------------------------------- ---------------------------- ,,footnotes3 #a ,the word medium is employed by physicists to express any substance through which light passes1 and includes solids such as glass1 liquids such as water1 and gases such as air2 the nature of the substance is not postulated4 #b ,methylene iodide must be heated almost to boiling-point to enable it to absorb sufficient sulphur2 but caution must be exercised in the operation to prevent the liquid boiling over and catching fire1 the resulting fumes being far from pleasant4 ,it is advisable to verify by actual observation that the liquid is refractive enough not to show any shadow-edge in the field of view of the refractometer4 #c 1 angle2 1 measure4 ,for details of the construction1 adjustment1 and use of this instrument the reader should refer to textbooks of mineralogy or crystallography4 #d ,a cleavage flake of topaz may conveniently be used to show the phenomenon1 but owing to the great width of the angle the 8eyes0 are invisible4 #e ,in accordance with the usual custom the angle between the facets is taken as that between their normals1 or the supplement of the salient angle4 #f ,the word paste is derived from the ,italian1 pasta1 food1 being suggested by the soft plastic nature of the material used to imitate gems4 #g ,cf4 below1 p4 #adi4 #h ,the least and the greatest of the refractive indices of doubly refractive species are given4 #i ,the dispersion is the difference of the refractive indices corresponding to the ,b and ,g lines of the solar spectrum4 ,the value for crown-glass is #jaf4 ---------------------------------------- ---------------------------- ,,interesting ,,and ,,important ,,books ,,jewellery4 ,by ,,cyril ,,davenport1 ,f4,s4,a4 ,with a ,frontispiece in ,colour and #da other ,illustrations4 ,second ,edition4 ,demy #afmo4 ,little ,books on ,art4 ,,jewellery4 ,by ,h4 ,,clifford ,,smith1 ,m4,a4 ,with #ej ,plates in ,collotype1 #d in ,colour1 and #cc ,illustrations in the text4 ,second ,edition4 ,wide royal #hvo1 gilt top4 ,connoisseur's ,library4 ,,goldsmiths' ,,and ,,silversmiths' ,,work4 ,by ,,nelson ,,dawson4 ,with #ea ,plates in ,collotype1 a ,frontispiece in ,photogravure1 and numerous ,illustrations in the text4 ,second ,edition4 ,wide royal #hvo1 gilt top4 ,connoisseur's ,library4 ,,european ,,enamels4 ,by ,h4 ,h4 ,,cunynghame1 ,c4,b4 ,with #eh ,illustrations in ,collotype and ,half-tone and #d ,plates in ,colour4 ,wide royal #hvo1 gilt top4 ,connoisseur's ,library4 ,,enamels4 ,by ,mrs4 ,,nelson ,,dawson4 ,with #cc ,illustrations4 ,second ,edition4 ,demy #afmo4 ,little ,books on ,art4 ,transcriber's ,notes3 - ,text enclosed by underscores is in italics 7italics74 - ,redundant title page has been removed4 - ,blank pages have been removed4 - ,front publication list moved to the back4 - ,silently corrected typographical errors4 - ,where possible ,unicode fractions have been used1 otherwise they are formatted as example 8#a-e_/af04