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glass used is known as paste or strass—heavy and transparent varieties of flint glass with high refractive indices and dispersion. The composition of the glass is as follows: 300 parts silica, 470 red lead, 163 K2CO3 22 of borax and 1 of white arsenic. The colouring pigments are added. The glass is ground and polished. The glass imitation has the following properties: it has inferior hardness than the gems they simulate and does not scratch ordinary glass, and exposed portions wear off soon; it is isotropic in character, singly refractive and does not show pleochroism. Refractive index depends on the composition—l.50-l.80; specific gravity 2-5-4-0; air bubbles are usually present but not seen in natural stones. Heat conductivity is low in glass imitations; and conchoidal fracture. The first four will decide the glass imitations. Most glass imitations of gems were made in Germany and Czechoslovakia.
BAKELITE AND OTHER PLASTICS. Bakelite has found a place amongst imitation gem-stones. It is a condensation product of phenol and formaldehyde. It could be made transparent, opaque or with bright inclusions. It is easily worked and takes a good polish. Imitations of amber and goldstones are common. They are easily moulded and worked and also possess wearing qualities.
DOUBLETS AND TRIPLETS. True doublets consist of two sections of genuine material to make a larger stone, which gives a larger value than that of 2 smaller stones. Its nature could be detected by immersing it in a liquid, whose refractive index is equal to that of the stone or soaking it in alcohol or chloroform in which the cement is dissolved.
THE FALSE DOUBLET. It has a genuine crown and pavilion of inferior stone or glass. A thin slice of ruby or garnet may be matched with coloured glass. It could be detected by the methods already mentioned. Triplets have been used to imitate the emerald. The top portion
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