Because of its vivid, opaque color, with hues from green to sky blue, turquoise has been a valued gem in many cultures, from the ancient Egyptians to the Aztecs and Native Americans. The archaeological evidence shows that it was probably the first gemstone that people ever extracted and also the first to be imitated. Turquoise, a copper aluminum phosphate, is generally soft, porous and easily subject to scratching and breaking. It is a sensitive gem, which needs protection from heat, bright light and chemicals.
Color is the most appreciated quality of turquoise. The most desired is the sky blue shade, which results from the presence of copper in the composition of the mineral. This hue is mostly extracted from the mines in Iran. The green turquoise shade is a product of iron and chrome, and is considered less precious. Color variations are so common that not all the stones from one mine will have the same hue.
Ochre or brown/black veins are common in turquoise, the results of oxide staining or rock fragments. The veins form patterns called "turquoise matrix." The stones with more uniformity of color are considered more valuable, followed by the spiderweb matrix pattern that looks clear and regular.
Production and Treatment
Gems found on the market vary from natural, stabilized and treated to reconstituted and synthetic turquoise. To enhance durability and color, the natural stone is generally impregnated with wax or synthetic resins and the result is called stabilized turquoise. Some gems may be treated with paint to improve coloration. There is turquoise reconstituted from the pulverized mineral, dye and resins. Synthetic stones are also marketed. Artificial gems might be sold under names like Viennese turquoise, Hamburger turquoise, or Neolith.
Cut and Shape
Because of its brittleness, turquoise is not generally faceted, but polished into cabochons (convex form) or beads.
One of the most important sources of turquoise is the Middle East. Extraction of this mineral represented Iran's main industry before World War I. The gem is also found in US mines, and other deposits have been exploited in Armenia, Kazakhstan, China, Australia, Tibet, Chile, Mexico and Brazil. The Iranian turquoise is most valuable because of its vivid blue shade, and the US extractions yield stones that are lighter in color, more porous and usually with matrix.
The oldest turquoise beads have been found in Mesopotamia (Iraq), dating from about 5000 B.C. The first imitations of turquoise (in the form of blue and green glazed soap-stone) were discovered in Egypt, and date from 3100 B.C.
The gem took its current name in the thirteenth century from the French appellation of "pierre turquoise", which means "Turkish stone." Before this, it was known as "callais," from the Greek "kalos lithos," meaning "beautiful stone."