The turquoise is called in chemical language a hydrated phosphate of alumina. This means that it consists mainly of phosphoric acid and alumina, along with nearly twenty per cent. of water. It owes its colour to small quantities of compounds of copper and iron. It occurs blue, green, and bluish green; but the change to a pale, mottled, or white colour, which inferior turquoises undergo, generally takes place soon after they are taken from the mine. These colours are opaque, or only very slightly translucent, and the stone has a somewhat waxy lustre. It is only those of a fast ‘sky-blue’ colour that are prized for jewelry; but at one time, a green turquoise was more highly valued than a blue one. Nowadays, however, people have no patience with either precious stones or precious metals that can be easily mistaken for those of inferior value. Either green felspar, which is of the same hardness, or malachite, which is softer, might be mistaken for green turquoise, and both are more common minerals. But there is hardly any other natural stone of the same, or even inferior, hardness that can be confounded with a blue turquoise. The material of some fossil teeth when coloured with phosphate of iron does, however, resemble it. Still, there need be no confusion, because this substance is softer. It is called odontolite or occidental turquoise; while the real stone is known by jewellers as the oriental turquoise. Odontolite is easily recognised under the microscope by the characteristic markings of dentine. Opaque blue glass can be made to imitate the turquoise; but the former differs in lustre and in the nature of its fracture. Turquoises are found in Tibet, China, and the neighbourhood of Mount Sinai; but, as has been already stated, the supply for jewellers’ purposes comes almost wholly from the celebrated Persian mines. Very little was known about these till a remarkably interesting and exhaustive Report upon them was recently furnished to the British Foreign Office by Mr A. Hontum Schindler, who was for a short time Director of the mines. They are situated in a range of mountains bounding on the north an open plain in the Bâr-i-Madèn district, thirty-two miles north-west of Nishâpûr, in the province of Khorassan. Botanists tell us that the brightest blue is seen on alpine flowers. If pure mountain air could be supposed to brighten the colour of a gem as well as a flower, there is no want of it where these turquoise veins occur. Their position is between five and six thousand feet above the level of the sea, and a strong north wind blows almost continually over the ridges of the hills, rendering the situation very healthy. Wheat, barley, and mulberry trees grow well on the slopes at the lower of these heights.