Of these colors the pure sky-blue or robin’s-egg blue is by far the most highly prized and is in fact the only standard color for the gem. Green is, however, the most common and the most lasting color of the mineral, and it is one of the faults of the gem that the blue shades often fade to green after being exposed to the light for a time. In a stone of first quality, however, especially a Persian Turquois, such fading of color is exceptional. A good Turquois also maintains its color in artificial light. The hardness of Turquois is 6, in the scale of which quartz is 7. It is therefore somewhat more easily scratched than other gems. Its specific gravity varies from 2.6 to 2.8, being about that of quartz. It does not fuse before the blowpipe, but turns brown and assumes a glossy appearance. By the copper of the Turquois the blowpipe flame is usually colored green. When heated in a closed glass tube the mineral turns brown or black and gives off water. Almost any of these tests will serve to distinguish true Turquois from stones used to imitate it. It has a conchoidal fracture and waxy lustre. On account of its opacity it is almost never cut with facets like most other gems, but in a round or oval form with convex surface. The pieces desirable for cutting rarely reach a large size so that big gems of Turquois are comparatively unknown. Much of the so-called Turquois used in former times was bone-turquois, or odontolite, made from fossil bone colored by a phosphate of iron. It is obtained mostly from the vicinity of the town of Simor, Lower Languedoc, France. It is sometimes known as Western or Occidental turquois, in distinction from the Oriental turquois, most of which came originally from Persia. It does not retain its color by artificial light as does true Turquois and may be further distinguished by giving off an offensive odor when heated, owing to decomposition of animal matter. Further, it is lighter than true Turquois and does not give a blue color with ammonia when dissolved in hydrochloric acid, as does true turquois.