Gem (Lat. gemma, a bud), the designation of precious stones prized for their brilliant lustre and splendid colors or perfect limpidity. They possess also a hardness which renders them susceptible of the highest polish, and capable of retaining unimpaired the forms into which they are cut and the lines or figures that may be engraved upon them. These properties, in connection with their rarity, have given to them the highest value of all substances. The principal gems are diamonds, rubies, and emeralds; the finest specimens of these are noticed under their respective titles. Other beautiful species of gems are the agate, chrysoberyl, chrysolite, garnet, sapphire, topaz, tourmaline, etc. These stones are not usually presented by nature in their full beauty; but they are found in the form of worn pebbles among the sands derived from the disintegration of the rocks in which they were originally contained. Some, however, are obtained crystallized in the matrix of quartz, calcareous spar, or other gangue of veins in which they were produced, or in geodes, of the dark cavities of which they made with other crystals a lining or incrustation.

The crystals may have the perfections of the stone fully developed, but art is not satisfied with the form, and this is almost always sacrificed to fit the stone to the shapes judged best suited to display its highest lustre. The rough pebble requires the work of the lapidary to develop its beauties. The processes to which the stones are subjected are described in the articles Diamond and Lapidary; the method of carving them to bring out from their differently colored layers figures in relief is described under Cameo; and gem engraving will be treated in this article.-To distinguish gems from each other and from their artificial counterfeits, a practised eye was formerly sufficient; but modern imitations are so perfect, that the tests of comparative hardness and of specific gravity are often required. The chemical tests which are usually employed to distinguish minerals cannot be applied to these stones on account of the injury they would occasion. The finest collection of gems in the world is that of the emperor of Russia. Siberia has proved a rich field for their production, and all precious stones found there belong to the crown. They are taken to Yekaterinburg, and being cut and polished in the government works, the choicest are selected for the imperial treasury.

Clarke in his account of his travels in Russia makes frequent reference to the abundance of fine gems met with in the cabinets and jewellers' shops, and states that in Moscow they were so much more highly prized than in western Europe, that the most costly gems were sometimes purchased in London to be deposited in Russian collections. Different Asiatic countries, particularly Hindostan, Pegu, and Ceylon, have been famous from the remotest periods for their rich gems; and in modern times Brazil, Peru, and south Africa have rivalled them in these productions. - Gem Engraving, known also as the glyptic art (Gr. to engrave), was skilfully practised in very remote times. In Exodus xxviii. 17-20, the following stones are designated as those upon which were engraved the names of the 12 children of Israel: sardius, topaz, carbuncle, emerald, sapphire, diamond, ligure, agate, amethyst, beryl, onyx, and jasper. At this early period, as we learn from verse 11 of the same chapter, engraving of signets, and upon the hardest stones, was practised. The Israelites, it is believed, acquired the art from the Egyptians, who are known to have made use of the lapidary's wheel and emery powder, and are supposed to have been acquainted with the diamond and the method of engraving other hard stones by means of it. The Assyrians and Babylonians were very skilful in engraving on gems, many of which have been found in the ruins of their cities. Many of their seals are most delicately and minutely ornamented with various sacred devices and with the forms of animals. The Greeks adopted the art, and practised it with the greatest zeal and success. Their works of the time of Alexander the Great are still the most perfect specimens.

The most distinguished among their earlier artists was Pyrgoteles, who alone was permitted to engrave the head of Alexander, as Apelles only was allowed to paint his portrait, and Lysippus to cast his image in bronze. During the reign of Augustus in Rome, Dioscurides from Aeolia in Asia Minor attained the highest eminence. His head of Io is regarded by some as the finest engraved gem in existence, and others almost equally beautiful are two busts of Augustus, a head of Demosthenes on an amethyst, and vari-ous mythological representations. Several of the most skilful artists of Greece established themselves at Rome under the emperors. "With the empire the art declined, and though the mechanical execution was not lost, no productions of genius appeared till the l5th century. At this time it became fashionable to make collections of antique gems, and among others Lorenzo de' Medici was especially interested in this object and in encouraging artists to imitate the finest productions, in which they attained great success. The same taste soon spread to France, Germany, and England: and in all civilized countries the art has since been held in high estimation.

As applied to the hardest gems, as the diamond, ruby, sapphire, and topaz, it is no doubt carried to a higher degree of perfection than was attained in ancient times, for among the antique engraVed gems preserved there are few if any of this class. Engraved gems are for the most part readily referred by connoisseurs to their true period, country, and sometimes to the artist himself. Each had his own cipher, which is commonly found upon the gem, though this is in modern times imitated, together with the peculiar style of the ancient artists and their complete work, in great perfection. Specimens of Egyptian art are recognized by the representations of the peculiar favorite animals and divinities of this people, accompanied with their hieroglyphics. Their gems were engraved almost exclusively in intaglio, and they were of the form designated as scarabaei, from the upper surface of the stone, always of the oval form, being cut to represent the beetle. The Etruscans also adopted this form, but their devices more resemble those of the earliest Greek workmanship.