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Gemstone Ruby Sapphire etc.

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CHAPTER VI.
IMITATIONS OF PRECIOUS STONES.
The one point in which all artificial imitations of precious stones fail is hardness. Practically they all yield to the file, and many are scratched even by a bit of common glass. Indeed, with rare exceptions, they consist of flint glass containing an unusually large proportion of lead and tinctured by the addition of certain colour­ing oxides, stich as cobalt for blue, manganese for violet, as well as nickel, copper, iron, chromium, or mixtures of these, for other hues. Colourless strass, as it is called, commonly contains 38 per cent, of silica, 53 oxide of lead, 8 potash, and traces of boracic and arsenious acid, with some alumina and soda. There are three other points in which these coloured glasses differ from true stones. Besides their softness already named, they tarnish in impure air, the lead becom­ing sulphided, and therefore brown ; they are heavier than any of the stones having specific gravity under 3*3, which they represent, and they are all destitute of pleochroism. Under the microscope, or even a hand magnifier, the majority of them show many lines, and specks, and air-bubbles, which betray their origin and nature —their origin, at a high temperature rapidly reduced ; their nature, as fused, glassy, non-crystalline masses. The lines and striae are signs of layers of unequal density and of strain ; the bubbles are rounded cavities, quite different from those cavities, with angular and crystalline walls, which some gem-stones, such as amethyst, beryl, topaz, frequently present. This is true not only of the many varieties of coloured paste or " strass," which form the usual 844.1.                                                                                                        l 2
Synthetic gemstones ruby and sapphire Page of 149 Gemstone Ruby Sapphire etc.
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