This mineral differs from nearly all others held in favor as gems, in not being transparent and never occurring in the form of well defined crystals. The opal is perhaps the only other gem of which the same may be said. In composition Turquois is a hydrous phosphate of aluminum, the percentages being: Of water, 20.6 per centum; of alumina, 46.8 per centum, and of phosphoric oxide, 32.6 per centum. Thus in composition as well as opacity Turquois differs from most other gems, they being usually silicates or some form of silica. Besides the above ingredients Turquois always contains a small percentage of copper oxide and usually iron, calcium and manganese oxides in small amount. It is the copper compound which undoubtedly gives Turquois its inimitable color, that color to which it owes its chief charm as a gem. The color varies from sky-blue through bluish-green and apple-green to greenish-gray.

Of these colors the pure sky-blue or robins 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.

The finest Turquoises have long come from Persia, from a locality not far from Nishapur, in the province of Khorassan. Here the mineral occurs in narrow seams in the brecciated portions of a porphyritic trachyte and the surrounding clay slate. There are several hundred mines in the region and the entire population of the town of Maaden derives its livelihood from mining and cutting the stones. It is said that $40,000 worth of stones are taken from these mines annually. A pound of stones of the first quality sells at the mines for about $400 and is worth more than double that price in Europe. There are other Turquois mines in Persia, but their product is comparatively small. "Persian Turquoises" have, however, the highest value of all. Other Oriental localities from which the gem Turquoises are obtained are Sinai, in Arabia, the Kirgeshi Steppes, in Siberia, and the Kara-Tube Mountains, in Turkestan. Egypt also furnishes large quantities of Turquois, which does not as a rule retain its color well.

Turquois is not an uncommon mineral in the United States and many gems of fine quality have been obtained from mines within our borders. The oldest and best known mines are those at Los Cerrillos, New Mexico. This locality .was long worked by Indians and Spaniards, as shown by the great extent of the excavations. There are pits two hundred feet in depth and piles showing that thousands of tons of rock have been broken out. Fragments of Aztec pottery, vases, cooking utensils, stone hammers, etc., are found at the mines, and trees of considerable size have grown over the once worked portions. Hence the beginning of the mine workings must at least date back prior to the discovery of America.