Gem
a precious stone. Gems are sometimes found crystallized in regular shapes and with a natural polish, more commonly of irregular shapes and with a rough coat. The term gem often denotes more particularly a stone that is cut, polished, or engraved, and it also includes pearls and various artificial productions. The first and most valuable class of gems includes diamonds, emeralds, rubies, sapphires, and a few others; the second class includes the amethyst, topaz, garnet, etc.; while agate, lapis-lazuli, cornelian, etc., though much used for ornament, can scarcely be called gems. The fabrication of artificial gems is now prosecuted with skill and capital, and has become an important industrial art. One class called semi-stones or doublets, are made by affixing thin slices of real gems to an under part of the strass by means of invisible cement. In some cases an imitation is made by setting uncolored strass or quartz in jewelry with some colored "foil" at the back of it. Attempts have been made with a fair measure of success to manufacture true gems by artificial processes. In 1858 MM. Deville and Caron communicated to the GEMARA Academy of Sciences, Paris, a process for the production of a number of gems of the corundum dum class , as rubies, sapphires, etc. The process essentially consisted in exposing the fluoride of aluminium, together with a little charcoal and boracic acid, in a plumbago crucible protected from the action of the air, to a white heat for about an hour. Many exexperiments with a view to producing diamonds artificially have also been made. In art and archæology the term gem is usually applied to a precious stone cut or engraved in ornamental designs, or with inscriptions. Stones on which the design is raised above the general surface are called cameos; those having the design sunk below the surface are called intaglios. Early specimens of cut gems are seen in the scarabæi or beetle-shaped signets worn in rings by the ancient Egyptians. Among the Greeks, Etruscans, and Romans gem sculpture held a high place, reaching its highest point under Augustus. Modern gem engraving dates from the beginning of the 15th century, the chief seats of the art being Italy and Germany. Rome is now the headquarters of the seal-engraving art.
The stones used for cameo cutting often exhibit layers of different colors, SO that different colors, so that the raised design has a tint distinct from the ground. Intaglios are very often executed in transparent stones, and the subjects treated in this manner are more limited in number. They are chiefly such as seals, devices, coats of arms, etc.
See DIAMOND.
GEMARA (ge-mä'rä), that portion of the two Talmuds which contains the annotations, discussions, and amplifications of the Mishnah by the academies of Palestine on the one hand, and those of Babylon on the other. The Babylonian Gemara, more complete as well as more lucid than the Palestinian, possesses a much more highly valued authority.
The final redaction of this latter falls in the middle of the 4th century A. D., while the former was not completed till 500 A. D. See TALMUD.