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 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 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 LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, GREAT WINDMILL STREET, W., AND DUKE STREET, STAMFORD STREET, S. E.