_Artificial Gems._--The fact that pieces of suitably-coloured glass can be made to show a superficial, but sometimes more or less deceptive, resemblance to precious stones, has led to the manufacture of imitation jewels of all descriptions. The glass used for this purpose is usually a very dense flint-glass whose high refractive index facilitates the imitation which is aimed at. The external shapes of gems are, of course, readily imitated by cutting and grinding the glass, while the requisite colours are attainable by means of the colouring materials described in Chapter XI. To a casual observer the difference in sparkle and brilliance which arises from the difference between the refractive index of the heavy flint-glass (about 1·8) and that of minerals (which ranges from 1·7 to 2·2) is not readily apparent, but closer examination will at once reveal the difference. The determination of the optical constants by means of a refractometer would at once reveal the true character of the imitation, but an even readier test is that of hardness. The dense flint-glass is naturally soft, and is readily scratched by most of the harder minerals, while the precious stones, more particularly garnets, rubies and diamonds, are very hard. If an attempt is made to scratch an ordinary sheet of window-glass, it will be found that most real precious stones will do so readily, while flint-glass imitations will fail to make more than a slight mark, which is more smear than scratch. The test by determining the specific gravity is also obviously applicable, since the flint-glass will readily betray its presence by its high density (over 4).
In quite a different class from the imitation gems made of cut flint-glass are the artificial gems, which in nature and composition are exact reproductions of natural gems, but which have been produced by artificial processes. As far as the writer is aware these are only found in any large numbers in the case of the ruby, but in that case, at all events, it is said that the production of the artificial crystals is at least as costly as the purchase of the natural stones. There can, however, be very little doubt that as the processes of fusion and crystallisation become better known and understood, and the chemistry of silicate minerals is developed, the artificial production of mineral crystals in, at all events, moderate sizes will become increasingly possible; it is even to be hoped that their production will be so far perfected as to place their really valuable properties at the service of man.