GELSENKIRCHEN (gel'zen-ker chen), a modern manufacturing town of Westphalia, 4 miles N. W. of Bochum. It owes to coal and iron its rise from a mere village since 1860. Pop. about 170,000.
GEN, 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 de notes 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, emer aids, 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 arti ficial gems is now prosecuted with skill and capital, and has become an impor tant 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 imi tation 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. De vine and Caron communicated to the Academy of Sciences, Paris, a process for the production of a number of gems of the corundum class, as rubies, sap phires, etc. The process essentially con
sisted in exposing the fluoride of alu minium, 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 ex periments with a view to producing dia monds artificially have also been made.
In art and archwology 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 sur face are called cameos; those having the design sunk below the surface are called intaglios. Early specimens of cut gems are seen in the scarabmi or beetle-shaped signets worn in rings by the ancient Egyptians. Among the Greeks, Etrus cans, 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 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.