GEMS, the name given to precious stones in general, but more especially to such as by their color, brilliancy, polish, purity, and rarity, are sought after as ob jects ,of decoration. Gems of the most valu able form the principal part of the crown jewels of sovereign princes, and are esteemed not merely for their beauty, 'out as comprising the greatest value in the smallest bulk. Gems are remarkable for their hardness and internal lustre. Un der this name arc comprehended the dia mond, ruby, sapphire, hyacinth, beryl, garnet, emerald, topaz, chrysolite, &e. To these hare been added rock crystals, the finer flints of pebbles, the cat's eye, the oculus mundi, the chalcedony, the moon-stones, the onyx, the cornelian, the sardonyx, agate, 3.e. Of most of these species there are some of an inferior class and beauty; these are commonly called by jewellers occidental stones. They are mostly the produce of Europe, and found in mines or stone quarries; and are so named in opposition to those of a higher class, which are always accounted ori ental; and supposed to be only produced in the east.— Gem-engraring, or gem sculpture, called also Uthog/yptics, is the art of representing designs upon precious stones, either in raised work, as cameos, or by figures cut below the surface, as intaglios. This art is of great antiquity, and was probably practised by the Baby lonians. Some think the art originated
in India; but wherever it originated, we have ample evidence that among the Greeks and Romans it was in high es teem. The merit of cameos and intag lios depends on their erudition, as it is termed, or the goodness of the workman ship, and the beauty of their polish. The antique Greek gems are the most highly prized; and, next to them, the Roman ones of the times of the higher empire.— Artificial gems. In order to approxi mate as near as possible to the brilliancy and refractive power of native gems, a basis, called a paste, is made from the finest flint glass, composed of selected materials, combined in different propor tions, according to the preference of the manufacturer. This is mixed with me tallic oxydes capable of producing the desired color.—The imitation of antique gems consists in a method of taking the impressions and figures of antique gems, with their engravings, in glass, of the color of the original gems. Great care is necessary in the operation, to take the impression of the gem in a very fine earth, and to press down upon this a piece of proper glass, softened or half melted at the fire, so that the figures of the impression made in the earth may be nicely and perfectly expressed upon the glass.