A gem, as I have said, is exceptionally hard and transparent, as the diamond and smaragdus, or it is exceptionally beautiful because it is adorned with pleasing or variable colors as most species of jaspis. Transparency, unusual beauty of color, luster and brilliancy are, in great part, responsible for the value. However, even though some congealed juices such as salt, nitrum, alumen and atramentum sutorium are transparent they cannot be numbered among the gems because they are not hard and for the same reason gypsum and silver-colored mica which are also transparent. Nor the stones which melt in a fire although they have the same colors and are as transparent as gems. Tephrites, diphyes, enorchis, cryptopetra, tecolithos and similar stones are not classed as gems because they cannot be cut, they are not brilliant, nor are they adorned with beautiful or variable colors. For the same reasons asbestos, bostrychites, corsoides, polia and spartopolios which are names for asbestos are not regarded as gems. On the other hand hematite, lysimachia, arabica, alabastrites, meroctes, obsidianus, siderites and similar stones are classed as gems because these stones as well as small fragments of marble are cut and polished and to a limited extent set in rings. However we will not treat these minerals here. A small piece of hematite does not differ from a large mass either in color or porperties, only in size. Lysimachia is the same as Rhodian marble; arabica, as Arabian marble; capniies, as marble with smoky spirals; alabastros, as alabastrites; exhebenus, as samius lapis; obsidiana . . .