December Birthstone - Turquoise

Physical and Chemical Properties

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Chemical composition

Turquoise is a hydrous phosphate of aluminum and copper CuO.3Al2O3.2P2O5.9H2O. Turquoise may be a weathering product of the mineral apatite, or it may even include some organic phosphate. Turquoise is fairly hard, 5-6 on a scale of 10. It may be chalky to brittle, the latter showing a conchoidal fracture. Better rough grades may have a waxy luster whereas lower grades are dull. The chalky material is often impregnated with a resin to make it more durable and more lustrous. The resin-treated turquois can usually be detected by odor. The specific gravity ranges from about 2.60 to 2.83. This December birthstone is almost always opaque but rare, translucent stones are known to exist. The refractive indexes range from 1.61 on the low end to 1.62 in the middle and 1.65 on the high side of the scale. It is biaxial positive and has a fairly high dispersion. Optical tests are usually not needed to determine if a stone is turquoise as physical tests will almost always suffice.

Colors

Turquoises are opaque unless they are found in very thin pieces. Colors include dark to light blue due to the copper and different ranges of green from iron.

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