|
December Birthstone - Turquoise
Physical and Chemical Properties
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.
Back to December Birthstones Summary Page
|
|