Turquoise
This article is about the gem. For the color, see Cyan.
gemstone. The highest grades are "robin's egg blue," or the color of the blue sky. Inferior grades are greenish. Turquoise which fades in color is also inferior. Turquoise is a phosphate of aluminium with small quantities of copper and iron. Chemically it is a hydrated copper aluminum phosphate: CuAl6(PO4)4(OH)8*5(H2O) with iron substituting for copper in variable amounts (high Fe content may give the more green colors). The gem is slightly harder than glass. The crystal class is triclinic, but most specimens are massive or cryptocrystalline, meaning the crystals are only visible under very high optical magnification or through X-ray diffraction studies.
Much modern turquoise jewelry is made from a variety of the mineral called "chalk turquoise" which is a light blue to green, porous chalky appearing mineral in its raw state. This chalky material is indeed turquoise as shown by x-ray studies. The material is pressure impregnated with plastic resin which gives it a deeper color and a consistency that can be worked and polished as a gemstone. The resulting stone has a very similar appearance to true gem turquoise, but of course is not as hard and would have a different density.
In ancient times, turquoise was used by the Egyptians and was mined by them in the Sinai Peninsula. There are important deposits in Iran near Nishapur and in the American Southwest. Turquoise was more or less the national stone of Persia and was used extensively to decorate objects. Turquoise is used by Native American craftsmen, especially Navaho silversmiths. Vivid material quarried in Arizona is known as Bisbee Blue.
Turquoise, along with coral is used extensively in the jewelry of Tibet and Mongolia. Turquoise is found in China and mined for the export trade, but not used for jewelry. A few carvings exist done in the same manner as jade carvings.
Further reading
- Turquoise, Volume XII, Part II, Second Memoir, Third Memoir, Memoirs of the National Academy of Sciences, Joseph E. Pogue, The Rio Grande Press, Glorieta, New Mexico, hardback reprint, about 200 pages, ISBN 0-87380-056-7
See also
List of minerals
