materials for imitative gems, but also of the fused compounds having the precise (or at least analogous) chemical composition of various gem-stones which have been prepared by Mr. Greville Williams and M. Feil. The green beryl glass of the former, and the blue lime spinel of the latter, afford cases in point.
Instead of substituting a wholly imitative preparation for a true stone, a doublet or triplet is constructed, in which a colourless or pale stone, of no value, is made to appear possessed of a fine deep colour. The doublet sapphire has a table and crown―all the stone down to the girdle―of colourless or pale blue sapphire, then the lower part of the combination, attached by cement, is made from blue glass or strass. If then the upper part of the stone be tested for hardness it answers to that of the sapphire, but if the base be examined, it immediately betrays its softness. To avoid this the triplet has been devised. Here we have pale sapphire for crown and base, but a thin layer of deep blue glass at the girdle―a part generally hid by the mount. To detect this imposture immersion in water generally suffices, for then the three layers will become visible; and if a doublet or triplet be boiled in water, or soaked in a small bottle of chloroform, it usually betrays its composite nature by falling to pieces. We should add that some false stones of this sort are coloured by means of a layer of coloured varnish or cement.
Imitation pearls claim a word of description. They are small spheres blown on tubes of slightly opalescent glass, and coated internally with a preparation made from the scales of a certain fish (as the bleak), and called Essence d'Orient. Into the little opalescent glass globe a coating of parchment size is introduced, and then a film of the pearl essence. Lastly, when the essence is dry, the bead is filled with wax. In order to produce an appearance like the orient of the true pearl the glass globes before filling are sometimes heated under pressure with a hydrochloric acid solution; in this way an iridescent surface effect is produced.
