06/16/2015
As early as 4,000 – 5,000 BCE humans, first in the Sinai region of Egypt, and a millennium later in Mesoamerica and China, were mining and working turquoise into jewelry and ceremonial objects. It was so highly valued in Eqypt, that when high quality deposits were exhausted, artisans developed a copper glazed ceramic simulant called faience, rather than abandon use of that sky blue color in their artwork.
Faience “Mummy” beads, circa 300 BCE
Chemically, turquoise is a hydrated copper/aluminum phosphate, of aggregate, cryptocrystalline structure. There is only one known deposit, in the state of Virginia, where turquoise is found in transparent to translucent visible crystals. Specimens from that locale are rare and bring a hefty price from collectors. More typically, turquoise is found as an opaque deposit in nodules, or veins within host rocks, or as shallow crusts on the surface of rocks.
Color ranges through shades of blue to blue-green, to yellowish green depending on the amount of copper, (adds blue,) chromium or vanadium, (adds green,) and iron, (adds yellow). There are rare specimens of blue-violet color which contain strontium impurities. In general, US mines produce slightly greenish blue, to green gems due to high iron and vanadium content. Most turquoise rough contains patches or veins of the host rock in which it formed, such as chalcedony or opal, brown limonite, black chert, or white kaolinite.
Such matrix can affect the color and toughness of the stone and its workability for the lapidary or jeweler. Relatively pure specimens of turquoise might have a hardness of around 5 and be moderately porous. In general, a high proportion of silicate minerals increases hardness and decreases the porosity, while a high content of clay minerals, has the opposite effect. On one end of this spectrum, then, we find pieces of hardness 5.5 to 6 that take a bright polish and are minimally porous, and on the other end are pieces of a soft and chalky nature with so much porosity as to be unusable without stabilization.
Turquoise occurs, usually in arid regions, where ground water percolates through aluminous rock in the vicinity of copper deposits. Like malachite, it is a secondary mineral which forms through the interaction of pre-existing minerals and their solutions. Historically the finest material was obtained from mines in Persia, (Iran,) and there is still considerable production from that area. The majority of today’s commerce in turquoise is primarily from North America and China. Its name, from French, means “Turkish stone”, a reference to the long history of imports of Persian material, through Turkey, to the West.
The US deposits are almost exclusively limited to the Southwest with Nevada home to a larger number of mines than Arizona, New Mexico and Colorado put together. This is a source of pride for Nevada and turquoise is, in fact, its official State Mineral.
Historically, and to a large extent today, the most admired stones are those of a fine robin’s egg, or celestial blue color, with no visible matrix, (this shade is an indication that no iron and little vanadium is present). Sometimes referred to as “Persian grade,” turquoise of this sort is still produced in Iran, but has been supplemented by similar stones from the US, particularly those obtained from the Sleeping Beauty Mine near Globe, Arizona.
In the Middle East it is traditional to set turquoise in gold, sometimes with diamonds. The Victorians also greatly admired turquoise, and generally set it in gold as well. In the US, though, turquoise has had a long historical association with silver jewelry
