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PRECIOUS STONES.
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In composition it is a hydrous phosphate of aluminium, iron and copper, and the formula
has been suggested by Penfield, who believes the copper and iron to be essential constituents. The colour is probably due to these metals in vary­ing proportion. It is infusible before the blowpipe, but on heating it decrepitates and gives off water. Most varieties are soluble in the mineral acids. Many speci­mens are found to lose their colour after a time, possibly owing to a spontaneous dehydration.
The most important locality is Persia, where it occurs in volcanic rocks intrusive into various sedimentary rocks in the Khorassan province, at the village of Maden, associated with Limonite. These mines were mentioned by Ben Mansur (1300), who also gives as localities for Turquois, Ghasna, Kerman and Irak. The material is not only found in situ, but also in detritus derived from these volcanic rocks, and in this débris some of the finest specimens are found. In addition it occurs at several other places in Persia.
In the Sinai Peninsula it is found in the Wadi Meghara in mines that have been worked, it is supposed, since 3000 b.c. In the Kirghiz Steppes it is found of a greenish blue colour. Also in the Kara-Tube Mountains in a siliceous rock, with Limonite. In the United States it is found at several places in New Mexico, and in all of these it seems to have been worked in remote times by the ancient Mexican races ; one of the principal places there is Mount Chalchuitl in the Los Cerillos Mountains ; another is in the Burro Mountains. In Nevada it is found north of Columbus ; also in Arizona and California.
Most of the searching for the material is now done in