Gem
2 of the 7 encyclopedias on this shelf carry an entry for Gem. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
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 few a 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 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 experiments 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 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.
Aiton's Encyclopedia (1910)
a precious stone. The use of the term implies that the stone has been cut and polished for ornamental purposes. The list of gems is a long one. The principal stones are the carnelian, chalcedony, onyx, sardonyx, agate, emerald, jasper, garnet, jacinth, ruby, beryl, amethyst, sapphire, lapis lazuli , topaz, turquoise, tourmaline, opal, and diamond. The art of engraving precious stones is as old as civilization itself. A gem cut so that a figure stands up is known as a cameo. In case the figures are cut out of the stone, so as to form depressions, the gem is an intaglio. Skilled workmen are able to imitate most gems in various sorts of pottery, porcelain, and glass. Separate articles may be consulted on the principal precious stones. As in the case of flowers, many gems have particular significance, the origin of which it would be difficult to trace. The astrologer claimed that the onyx was under the influence of Saturn; the carnelian, of Jupiter; the diamond, of Mars; the sapphire, of the Sun; the emerald, of Venus; the lodestone, of Mercury; and the crystal, of the Moon. In sentimental matters each month has its gem, and, like flowers, each gem has its significance. The following table carries with it at least the authority of long established tradition: January Constancy. February Sincerity. March Courage. April Innocence. May Success in love. June Health and long life. July Content. August Conjugal felicity. September Antidote for madness. October Hope. November Fidelity. December Prosperity.