OF all the opaque stones turquoise (Plate XXIX, Fig. 17) alone finds a prominent place in jewellery and can aspire to rank with the precious stones. The colour varies from a sky-blue or a greenish blue to a yellowish green or apple-green. Only the former tints, which are at the same time the rarer, are in general demand, and they possess the great advantage of harmonizing with the tint of the gold setting. The blue colours are, especially in the case of the Siberian stones, by no means permanent, and fade in course of time. Turquoise is amorphous and seldom crystalline, and is therefore somewhat porous; it should consequently never be immersed in liquids or be contaminated with greasy and dirty matter lest the dreaded change of colour be brought about. The stones are trans-lucent in thin sections, and a good observation is possible with the refractometer if the back of the stone is flat and polished, since only the section immediately adjacent to the instrument is concerned ; the refractive index is about 1.61. The specific gravity varies from 2.75 to 2.89. Turquoise has a hardness of slightly under 6 on Mohs’s scale, and takes a good polish, which is fairly durable, since on account of the comparative opacity of the stones scratches on the surface are not very noticeable. In composition it is a complex phosphate of aluminium and copper, corresponding to the formula CuOH.[6Al(OH)2].H5.(PO4)4, with ferric oxide replacing some alumina. The blue colour is due to the copper constituent, and the predominance of iron may cause the greenish shades ; but the water contained in the stones plays no mean part, since they turn a dirty green when it is driven off. The faded colour can sometimes be restored by immersion of the stone in ammonia and subsequent application of grease, but the effect is not lasting. Attempts are sometimes made to improve inferior stones by impregnating them with Berlin blue, but with only qualified success. Turquoises are said to be affected by the perspiration from the skin.
The name of the species comes from a French word meaning Turkish, and arises from the fact that the gem-stone first reached Europe by way of Turkey. Another, but less obvious, suggestion is that it is derived from the Persian name for the species, piruzeh. Our turquoise and other phosphates of similar appearance were probably known to Pliny under the three names callais, callaina, and callaica.
The finest turquoise still comes from the famous mines near Nishapur in the Persian province of Khorassan, where it was known in very ancient times; it is found with limonite filling the cracks and cavities in a brecciated porphyritic trachyte. Pieces of the turquoise and limonite from here are sometimes cut without removal of the latter, and sold as ‘ turquoise-matrix,’ when the precious stones are too tiny to be worth separate working. It also occurs at Serbal in the Sinai Peninsula. Among the more recent localities may be mentioned Los Cerillos Mountains, New Mexico ; Sierra Nevada, Nevada, where pale blue and green stones are found ; San Bernardino County, California, where again the stones are rather pale; and Arizona, where it occurs in pale greenish-blue stones.
Some of the stones that have been seen are not the true turquoise but odontolite, or bone turquoise, which consists of the teeth and bones of mastodon or other extinct animals, phosphate of iron being the colouring material. These stones may easily be recognized by their organic structure, which is clearly visible if viewed with a strong lens or under the microscope. Moreover, odontolite invariably contains some calcium carbonate, and effervescence takes place if it be touched with hydrochloric acid. Turquoise dissolves in hydrochloric acid, but without effervescence, and since it contains copper, a fine blue colour is imparted to the solution by the addition of ammonia. Odontolite has a higher specific gravity, 3.0 to 3.5, but lower hardness, 5 on Mohs’s scale.
Variscite, the hydrated phosphate of aluminium, corresponding to the formula A1PO4+ 2 H20, is found in masses resembling a greenish turquoise, but it is much softer, being only 4 on Mohs’s scale. The specific gravity is 2.55. Round nodular masses of variscite are found in Utah.