Gem, a general name applied to all precious stones, which are divided into two classes; 1. The pellucid, or such as are clear, elegant and beautiful fossils, extremely hard, and of uncommon lustre ; 2. The semi-pellucid gems, which are found in small detached pieces, and are composed of crystalline matter debased by earth: they are, nevertheless, of great beauty and brightness, and somewhat transparent.
The value of gems depends principally on their hardness and colour. With respect to the former, the diamond is allowed to be the firm-est, and can only be polished, or cut, by its own powder : next to if, the ruby, sapphire, jacinth, emerald, amethyst, garnet, onyx, jasper, agate, porphyry, and marble are classed in the order we have enumerated. The same classfica-tion prevails in point of colour : the diamond is universally esteemed on account of its brilliancy; the ruby for its purple; the sapphire for its blue,; the emerald for its green; the jacinth for its orange ; the amethyst for its carnation ; the onyx for its tawney; the jasper, agate, etc. for their vermillion, green, and variegated colours; and the garnet for its transparent red.
The art of imitating gems is very difficult to be attained ; and, as it| can be practised only by those curious persons, who possess both leisure and means, we shall not enter into a detail. The same apology will apply to the imitation of what are called antique-gems; many valuable impressions of which have been made by Mr. Tassie : hence we cannot omit to mention Mr. Raspe's "Account of the Present State and Arrangement of Mr. James Tassie's Collection of Pastes,''etc. 8vo. 1786, where the inquisitive reader will find an interesting subject judiciously treated and explained.