Precious Stones/Chapter 6: 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, such 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 striæ 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 ​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.

​Some remarks on the artificial colouring of natural stones will be found in chapter vii.; the different varieties of silica―agate, onyx, cornelian, and even opal―are frequently subjected to processes of heating and saturation with chemical reagents in order to change their hue or to introduce foreign colouring matters.