GEM, in natural history, a common name for all precious stones; of which there are two classes, the pellucid and semipellucid.
The bodies composing the class of pellucid gems are bright, elegant, and beautiful fossils, naturally and essentially compound, ever found in small detached masses, extremely hard, pellucid, and of great lustre; composed of a very firm and pure matter, without any admixture of earthy substance; giving fire with steel, not fermenting with acid menstruum, and very difficultly calcineable in the fire. Of this class there are two genera; the chrostaïma, and the chrostaïtes. See CHROSTAÏMA and CHROSTAÏTES.
The bodies composing the class of semipellucid gems, are stones naturally and essentially compound, not inflammable nor soluble in water, found in detached masses, and composed of crystalline matter, debased by earth: however, they are but slightly debased, and are of great beauty and brightness, of a moderate
Gem. degree of transparency, and are usually found in small masses.
Of this class there are two orders: the first of which consists of the semipellucid gems, of but two variegations, and frequently of one plain simple colour; tho' sometimes veined: this order contains four genera, viz. the sardæ, the chalcedonies, the hydrophanæ, and the pramnion. See the articles SARDÆ, CHALCEDONII, HYDROPHANÆ, and PRAMNION.
The second order of semipellucid gems, consisting of those remarkable for their veins, zones, and variegations, contains also four genera, viz. the achate, the onyches, the sardonyches, and the camææ. See the articles ACHATE, ONYCHES, SARDONYCHES, and CAMÆÆ.
The knowledge of gems depends principally on observing their hardness and colour. Their hardness is commonly allowed to stand in the following order: the diamond the hardest of all; then the ruby, sapphire, jacinth, emerald, amethyst, garnet, carnel, chalcedony, onyx, jasper, agate, porphyry, and marble. This difference, however, is not regular and constant, but frequently varies. Good crystals may be allowed to succeed the onyx; but the whole family of metallic glassy stones seem to be still softer.—In point of colour, the diamond is valued for its transparency, the ruby for its purple, the sapphire for its blue, the emerald for its green, the jacinth for its orange, the amethyst carnel for its carnation, the onyx for its tawny, the jasper, agate, and porphyry for their vermilion, green, and variegated colours, and the garnet for its transparent blood-red.
All these gems are sometimes found coloured and spotted, and sometimes quite limpid and colourless. In this case the diamond-cutter or polisher knows how to distinguish their different species by their different degrees of hardness upon the mill. For the cutting or polishing of gems, the fine powder of the fragments of those that are next in degree of hardness is always required to grind away the softer; but as none of them are harder than the diamond, this can only be polished by its own powder.