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Chemistry, Properties and Tests of Precious Stones

<h2>CHAPTER XV.</h2> <h3>VARIOUS PRECIOUS STONES—continued.</h3> <h4>Zircon.</h4> Zircon appears to have been first discovered by Klaproth in 1789, in the form of an earth, and six years later he found that the stone hyacinth contained a similar substance, both having the formula, ZrSiO4, and both having as their colouring agent ferric oxide. There are several methods of obtaining the metallic element, zirconium; it is however with the silicate of zirconium that we have to deal at the moment. This is called zircon, ZrSiO4, or hyacinth when transparent or red, but when smoke-coloured, or colourless, it is the jargoon, or jarcon, and is found in silt and alluvial soils, limestone, gneiss, and various forms of schist, in India, Australia, the Urals, and certain parts of America. It is often combined with and found in juxtaposition to gold and certain varieties of precious stones. The lines of cleavage are parallel to the sides of the prism, and the crystals have an adamantine, or diamond lustre, varying from the completely opaque to the transparent. In some varieties the oxide of uranium is also present in traces. It crystallises in the 3rd (tetragonal) system, with indistinct[Pg 99] cleavage. Its specific gravity varies from 4.70 to 4.88, according to the specimen and the locality. This stone, like some of the others described, has a very wide range of colour, going through reds, browns, greens, yellows, oranges, whites, greys, blues from light to indigo, notwithstanding which it is somewhat difficult to imitate scientifically, though its composition of 33 per cent. of silica with 67 per cent. of zirconia (the oxide of zirconium), is practically all it contains, apart from the colouring matter, such as the metallic oxides of iron, uranium, etc. Its hardness is 7-1/2, consequently it is untouched by a file, and so far, if one or perhaps two of the three qualities of colour, hardness, and specific gravity, are obtained in a chemically made zircon, the third is wanting. Under the blowpipe, zircons are infusible, but the coloured stones when heated strongly become heavier, and as they are contracting, their colour fades, sometimes entirely, which changes are permanent, so that as they possess the adamantine lustre, they are occasionally cut like a diamond, and used as such, though their deficiency in fire and hardness, and their high specific gravity, make them readily distinguishable from the diamond. On exposure to light the coloured zircon becomes more or less decoloured; especially is this so in sunlight, for when the direct rays of the sun fall upon it, the colours fade, and for a moment or two occasional phosphorescence follows, as is the case when the stone is warmed or heated in a dark room. The stone appears to be very susceptible to brilliant light-rays, and in certain specimens which were split for testing, one half of each being kept excluded from light for purposes of[Pg 100] comparison, it was found that sunshine affected them most; then brilliant acetylene gas, which was more effective still when tinted yellow by being passed through yellow glass. The electric arc was not so effective, but the electric light of the mercury-vapour lamp, though causing little change at the first, after a few hours' exposure rapidly bleached certain of the colours, whilst having no effect on others. Coal gas with incandescent fibre mantle was slightly effective, whilst the coal-gas, burned direct through an ordinary burner, affected very few of the colours, even after twenty-four hours' exposure at a distance of three feet. In all these cases, though the colours were slightly improved by the stones being kept for a time in the dark, they failed to recover their original strength, showing permanent loss of colour. <h4>The Silicates.</h4> The chief of these are the garnets, crystallising in the cubic system, and anhydrous. The garnet is usually in the form of a rhombic dodecahedron, or as a trisoctahedron (called also sometimes an icosatetrahedron), or a mixture of the two, though the stones appear in other cubic forms. In hardness they vary from 6-1/2 to 8-1/2. They average from 40 to about 42 per cent. of silica, the other ingredients being in fairly constant and definite proportions. They are vitreous and resinous in their lustre and of great variety of colour, chiefly amongst reds, purples, violets, greens, yellows and blacks, according to the colouring matter present in their mass. There are many varieties which are named in accordance with one or more of their constituents, the best known being:[Pg 101] (A) The iron-alumina garnet, having the formula 6FeO, 3SiO2 + 2Al2O3, 3SiO2. This is the "precious" garnet, or almandine, sometimes called the "Oriental" garnet; these stones are found in Great Britain, India, and South America, and are deep red and transparent, of vitreous lustre. They get up well, but certain varieties are so subject to defects in their substance, brought about by pressure, volcanic action, and other causes, some of which are not yet known, that their quality often becomes much depreciated in consequence. This inferior variety of the iron-alumina garnet is called the "common" garnet, and has little lustre, being sometimes opaque. The perfect qualities, or almandine, as described above, are favourite stones with jewellers, who mount great quantities of them. The second variety is the (B) lime-iron garnet, formula, 6CaO,3SiO2 + 2Fe2O3,3SiO2. The chief of this class is the melanite, sometimes dull, yet often vitreous; it is mostly found in volcanic rocks, such as tuff; this variety is very popular with jewellers for mourning ornaments, for as it is a beautiful velvet-black in colour and quite opaque, it is pre-eminent for this purpose, being considerably less brittle than jet, though heavier. Another variety is the "topazolite," both yellow and green. The "aplome" is greenish-yellow, yellowish-green, brown, and usually opaque. A further form of lime-iron garnet is the "pyreneite," first found in the Pyrenees Mountains, hence its name. The (C) lime-chrome garnets—6CaO,3SiO2 + 2Cr2O3, 3SiO2—the chief of which is "uwarowite." This is of a magnificent emerald green colour, translucent at edges[Pg 102] and of a vitreous lustre. When heated on the borax bead it gives an equally beautiful green, which is, however, rather more inclined to chrome than emerald. This is an extremely rare stone in fine colour, though cloudy and imperfect specimens are often met with, but seldom are large stones found without flaws and of the pure colour, which rivals that of the emerald in beauty. The fourth variety (D) is the lime-alumina garnet, its formula being—6CaO,3SiO2 + 2Al2O3,3SiO2. Like the others, it has a number of sub-varieties, the chief being the "cinnamon stone," which is one of great beauty and value when perfect. This stone is almost always transparent when pure, which property is usually taken as one of the tests of its value, for the slightest admixture or presence of other substances cloud it, probably to opacity, in accordance with the quantity of impurity existent. This variety is composed of the oxides of aluminium and silicon with lime. In colour it ranges from a beautiful yellowish-orange deepening towards the red to a pure and beautiful red. "Romanzovite" is another beautiful variety, the colour of which ranges through browns to black. Another important variety is the "succinite," which gets up well and is a favourite with jewellers because of its beautiful, amber-like colour, without possessing any of the drawbacks of amber. (E) The magnesia-alumina garnet—6MgO,3SiO2 + 2Al2O3,3SiO2—is somewhat rare, the most frequently found being of a strong crimson colour and transparent. This variety is called "pyrope," the deeper and richer tints being designated "carbuncle," from the Latin carbunculus,[Pg 103] a little coal, because when this beautiful variety of the "noble" garnet is held up between the eyes and the sun, it is no longer a deep, blood-red, but has exactly the appearance of a small piece of live or glowing coal, the scarlet portion of its colour-mixture being particularly evident. The ancient Greeks called it anthrax, which name is sometimes used in medicine to-day with reference to the severe boil-like inflammation which, from its burning and redness, is called a carbuncle, though it is more usual to apply the word "anthrax" to the malignant cattle-disease which is occasionally passed on to man by means of wool, hair, blood-clots, etc., etc., and almost always ends fatally. A great deal of mystery and superstition has always existed in connexion with this stone—the invisibility of the bearer of the egg-carbuncle laid by a goldfinch, for instance. (F) The manganese-alumina garnet—6MnO,3SiO2 + 2Al{2}O3,3SiO2—is usually found in a crystalline or granular form, and mostly in granite and in the interstices of the plates, or laminæ, of rocks called schist. One variety of this, which is a deep hyacinth in colour, though often of a brown-tinted red, is called "spessartine," or "spessartite," from the district in which it is chiefly found, though its distribution is a fairly wide one. <h4>The Lapis-Lazuli.</h4> The lapis-lazuli, sometimes called "azure stone," is almost always blue, though often containing streaks of white and gold colour, the latter of which are due to the presence of minute specks or veins of iron pyrites, the former and colourless streaks being due to free lime,[Pg 104] calcite, and other substances which have become more or less blended with the blue colour of the stone. It has a vitreous lustre, crystallises in the 1st, or cubic system, and is a complex substance, varying considerably in its ingredients in accordance with the locality in which it is found, its matrix, and the general geological formation of the surrounding substances, which may, by the penetration of moisture, be brought to bear upon the stone, thus influencing to a great extent its chemical composition. So that we find the stone composed of about a quarter of its substance of alumina, or oxide of aluminium, silica to the extent of almost half, the remainder being lime, soda, sulphur, and occasionally traces of copper and iron. It is mostly found in granite and certain crystalline limestone rocks, in fairly large masses. It is of great antiquity, figuring extensively in ancient Egyptian history, both in its form as a stone and ground up into a pigment for the decoration of sacred and royal vessels and appointments. When so ground, it forms the stable and magnificent colour, genuine ultramarine, which is the finest and purest blue on the artist's palette, but owing to its extremely high price its use is not in very great demand, especially as many excellent chemical substitutes of equal permanence are obtainable at little cost. <h4>The Turquoise.</h4> The turquoise is a pseudomorph (see Chapter IV., "Cleavage.") In colour it is blue or greenish-blue, sometimes opaque, varying between that and feeble translucency, though it should be said that in all forms, even those considered opaque, a thin cutting of the stone[Pg 105] appears almost transparent, so that the usual classing of it among the opaque stones must be done with this reservation. In composition it contains about 20 per cent. of water, about a third of its substance being phosphoric acid, or phosphorus-pentoxide; sometimes nearly half of it is alumina, with small quantities of iron in the form of variously coloured oxides, with oxide of manganese. The great proportion of water, which it seems to take up during formation, is mostly obtained in the cavities of weathered and moisture-decomposing rocks. Its average formula may be said to be Al2O3P2O5 + 5H2O, and sometimes Al2O3 FeOP2O5 + 5H2O. It must therefore follow that when the stone is heated, this water will separate and be given off in steam, which is found to be the case. The water comes off rapidly, the colour of the stone altering meanwhile from its blue or blue-green to brown. If the heat is continued sufficiently long, this brown will deepen to black, while the flame is turned green. This is one of the tests for turquoise, but as the stone is destroyed in the process, the experiment should be made on a splinter from it. This stone is of very ancient origin, and many old turquoise deposits, now empty, have been discovered in various places. History records a magnificent turquoise being offered in Russia for about £800 a few centuries ago, which is a very high price for these comparatively common stones. Owing to the presence of phosphorus in bones, it is not uncommon to find, in certain caves which have been the resort of wild animals, or into which animals have fallen, that bones in time become subjected to the oozing[Pg 106] and moisture of their surroundings; alumina, as well as the oxides of copper, manganese and iron, are often washed across and over these bones lying on the cave floor, so that in time, this silt acts on the substance of the bones, forming a variety of turquoise of exactly the same composition as that just described, and of the same colour. So that around the bones there eventually appears a beautiful turquoise casing; the bone centre is also coloured like its casing, though not entirely losing its bony characteristics, so that it really forms a kind of ossified turquoise, surrounded by real turquoise, and this is called the "bone turquoise" or "odontolite." <hr style="width: 65%;" /> [Pg 107] <h2>INDEX</h2> Adamantine lustre, 28 glimmering, 29 glinting, or glistening, 29 lustreless, 29 shining, 29 splendent, 29 Agate, 11 Almandine, 101 Amethyst, 11 oriental, 85 sapphire, 85 Amorphous stones and their characteristics, 23 Analysis, 5 Aplome, 101 Asters, or asteriated stones, 82, 87-91 Azure-stone, 103 Beryl, 10, 94 colours of, in dichroscope, 34 Beryllium, 10 Bezils, 66 Black stones, list of, 79 Blue sapphire, composition of the, 10 stones, list of, 77 Bone-turquoise, 106 Break, as opposed to cleavage, 19 Brilliant-cut stones, 66 Brown stones, list of, 76 Building up of crystals, 13 Burnt, or pinked topaz, 92 Cabochon-cut stones, 64 (the double), 65 (the hollow), 65 Carbonate series, 11 Carbuncle, 102, 103 [Pg 108]Cat of Egypt, 89 Cat's eye stones, 82, 87-91 list of (see "Chatoyant Stones"), 78 Ceylonese cat's eye (see "Cat's eye") Change of colour (not to be confused with "Play of colour" and "Opalescence," which see; see also "Fire"), 36 Characteristics of precious stones, 1, 3 Chatoyant stones, list of, 78 Chemical illustration of formation of precious stones, 8 Chloride of palladium in dichroscope, 34 Chrysoberyl, 88 Chrysolite, 11 ordinary, or "noble", 85 oriental, 85 Cinnamon stone, 102 Claims of precious stones, 4 Cleavage affecting tests, 43 and "cleavage" as opposed to "break", 19, 22 Colour, 26, 28, 30, 32 Colourless stones, list of, 75 Colours and characteristics of the various opals, 35, 36 of precious stones, list of, 75-79 Common garnet, 101 opal, 35 Composite crystals, 13 Composition of paste, or strass, for imitation stones, 71 Composition of precious stones, 7 Converted stones, 72 Corundum, 82 Crown portion of stones, 65, 66 Crystalline structure, physical properties, of 13 Crystallography, 14 Crystals, axes of symmetry, 15 groups of, 15, 16 planes of symmetry, 15 systems of, 16 (1) Cubic—isometric, monometric, regular, 16 (2) Hexagonal—rhombohedral, 16 (3) Tetragonal—quadratic, square prismatic, dimetric, pyramidal, 16 (4) Rhombic—orthorhombic, prismatic, trimetric, 16 (5) Monoclinic—clinorhombic, monosymmetric, oblique, 16, 17 (6) Triclinic—anorthic, asymmetric, 16, 17 treatment of, 14 [Pg 109]Culasse portion of stones, 66 Cullinan diamond (see also "Stars of Africa"), 22, 64, 68, 80 Cutting of precious stones, 3, 4, 62 Cymophane, 90 Definition of a precious stone, 1 Diamond, characteristics of the, 80 composition of the, 10 (sapphire), 86 unique, 10 (zircon), 99 Diaphaneity, 26, 28 Diaphanous stones, 28 Dichroscope, 33 how to make a, 33 how to use a, 34 Dimorphism in precious stones, 25 Double cabochon-cut stones, 65 refraction (see "Refraction") Doublets, 72 Electric and magnetic influences, 57 experiments with precious stones and pithball and electroscope, 57 experiments with tourmaline, 58, 59 Emerald, 10, 11, 95, 96 oriental, 85 En cabochon-cut stones, 64 Experiments to show electric polarity, 58, 59 Facets in stones, description of the, 67, 68 Feminine stones, 85 Fire in stones (see also "Change of Colour," "Opalescence," and "Play of Colour"), 36, 37 Fire opal, 35 Flame-coloured stones, list of, 76 Flaws, 63 Formation of precious stones, 5, 8 chemical illustration of, 8, 9 Garnet, 11, 100 Garnets (A) iron-alumina (called also almandine and precious or oriental garnet), 101 [Pg 110]sub-variety, common garnet, 101 (B) lime-iron, 101 sub-variety aplome, 101 melanite, 101 pyreneite, 101 topazolite, 101 (C) lime-chrome, 101, 102 sub-variety uwarowite, 101, 102 (D) lime-alumina, 102 sub-variety cinnamon stone, 102 romanzovite, 102 succinite, 102 (E) magnesia-alumina, 102, 103 sub-variety carbuncle, or anthrax, 102, 103 noble, 103 pyrope, 102 (F) manganese-alumina, 103 sub-variety spessartine, or spessartite, 103 Girdle portion of a stone, 66 Glimmering, in lustre, definition of, 29 Glinting, or glistening in lustre, definition of, 29 Goutte de suif-cut stones, 65 Great Mogul diamond, 64 Green stones, list of, 78 Groups of crystals (see "Crystals") Hardness, physical properties of, 39 table of, 39, 40, 41 Heat indexes, 54 physical properties of, 52 Hollow-cabochon, 65 Hyacinth, ordinary (a form of zircon), 85, 98 oriental, 85 Hyalite (opal), 35 Hydrophane (opal), 35 Imitations and tests of precious stones, 70 Indigo sapphires, 86 Ink sapphires, 85 Iridescence, and cause of, 37, 38 Iron-alumina garnets, 101 Jacinth, oriental, 85 [Pg 111]Jarcon, or jargoon, 98 Koh-i-nûr, 64 Lapis-lazuli, 103 Light, physical properties of, 26 Lime-alumina garnets, 102 cinnamon stone, 102 romanzovite, 102 succinite, 102 Lime-chrome garnets, 101, 102 uwarowite, 101, 102 Lime-iron garnets, 101 aplome, 101 pyreneite, 101 topazolite, 101 List of stones according to colour, 75-79 hardness, 39-41 specific gravity, 48-50 Lustre, 26, 28 Lustreless, definition of, 29 Lynx-eye stones, 87 Magnesia-alumina garnets, 102, 103 carbuncle, or anthrax, 102 noble, 103 pyrope, 102 Magnetic and electric influences, 57-61 Malachite, 11 Manganese-alumina garnets, 103 spessartine, or spessartite, 103 Masculine stones, 85 Melanite, 101 Menilite (opal), 36 Metallic-lustre stones, 28, 29 Mohs's table of hardness, 39-41 Noble garnet, 103 or precious opal, 35 Non-diaphanous stones, 28 Odontolite, 106 Olivine corundum (see "Chrysolite"), 85 Opal, 11 varieties of, 35, 36 Opalescence (not to be confused with "Change of Colour" and "Play of Colour," [Pg 112]which see; see also "Fire"), 36, 37 Oriental amethyst, 85 cat's eye (see "Cat's eye") emerald, 85 garnet, 101 topaz, 85 Origin of precious stones, 7 Paste, or strass, for imitation stones, composition of, 71 Pavilion portion of cut stones, 66 Pearly-lustre stones, 28, 29 Peridot (see "Noble Chrysolite"), 85 Pink-coloured stones, list of (see also Red), 77 Pinked topaz, 92 Phosphorescence, 26, 30 Physical properties:— A.—Crystalline structure, 13 B.—Cleavage, 19 C.—Light, 26 D.—Colour, 32 E.—Hardness, 39 F.—Specific gravity, 45 G.—Heat, 52 H.—Magnetic and electric influences, 57 Play of colour (not to be confused with "Change of Colour" and "Opalescence," which see; see also "Fire"), 36, 37 Pleochroism, 33 Polarisation, electric, 58, 59 of light, 26, 27 Polariscope, 27, 28 Polishing precious stones, 3, 4 Polymorphism in precious stones, 25 Precious, or noble opal, 35 Pseudomorphism in precious stones, 23, 24 Pyreneite, 101 Pyro-electricity, development and behaviour of, 58-60 Pyrope, 102 Qualities of precious stones, 1, 3 Red and rose-coloured stones, list of (see also Pink), 76, 77 Reflection of light, 26, 28 Refraction of heat, 52-55 light, 26, 27 Reproduction of crystalline form, 20, 21 [Pg 113]Resinous lustre stones, 28, 29 Rock-crystal, 11 Romanzovite, 102 Rose-coloured stones (see Red, above), 76, 77 Rose, or rosette-cut stones, 65 Rothschild's testing solution, 73 Ruby, characteristics of, 83 composition of, 10 Sapphire, amethyst, 85 and its varieties, 84, 85 cleared, 86 diamonds, 87 indigo, 86 ink, 85 the blue, composition of, 10, 85 water, 86 Semi-diaphanous stones, 28 Shining, in lustre, definition of, 29 Silica group, composition of the, 11 Silicates, 100 Silky-lustre stones, 28, 29 Single-refraction (see "Refraction") South African diamond (see "Cullinan Diamond") Specific gravity, 45 Splendent, in lustre, definition of, 29 Splitting of the Cullinan diamond, 22 Star-portion of stones, 65 Stars of Africa (see also "Cullinan Diamond"), 22, 64, 68 Starting or splitting of stones on cleavage planes, 23 Step-cut stones, 66 Stones arranged according to colour, 75-79 hardness, 39-41 specific gravity, 48-50 Strass for imitation stones, composition of, 71 Sub-metallic in lustre, definition of, 29 Sub-translucent stones, 28 Sub-transparent stones, 28 Succinite, 102 Synthesis, 5 Systems of crystals (see "Crystals") Table-cut stones, 65 Tallow drops, 65 Teeth of stone, 65 Testing by crystalline structure, 17 [Pg 114]hardness, 40, 43 with needles, 41 gems by dichroscope, 33, 34 solution (Rothschild's), 73 Tests of precious stones (general), 70 Topaz, 11, 91 colours of, in dichroscope, 34 oriental, 85 Topazolite, 101 Tourmaline, 96, 97 electric experiments with, 58, 59 Translucent stones, 28 Transmission of heat, 52-56 light, 26 Transparent stones, 28 Trap-cut stones, 66 Tri-morphism in precious stones, 25 Triplets, 72 Turquoise, 104 (bone), 106 composition of the, 11 odontolite, 106 Uwarowite, 101, 102 Violet stones, list of, 78 Vitreous-lustre stones, 28, 29 Water-sapphires, 86 White (paste) stones, 71 stones, list of, 75 Yellow stones, list of, 76 topaz, 92 Zircon, 10, 98 diamonds, 99 Zirconium, 10 <hr style="width: 65%;" /> LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, GREAT WINDMILL STREET, W., AND DUKE STREET, STAMFORD STREET, S. 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