Turquoise
Turquoise Information For your gem needs see our Recommended Suppliers.
WEARABILITY* Poor
ENHANCEMENTS
*Wearability is graded as Excellent, Very Good, Good, Poor, and Forget It!
CHEMISTRY CuAl6(PO4)4(OH)8 . 5H20 + Fe
CRYSTALLOGRAPHY Triclynic, although usually massive
REFRACTIVE INDEX 1.590 - 1.650
HARDNESS 5 to 6
SPECIFIC GRAVITY 2.40 - 2.90
CLEAVAGE None
HEAT SENSITIVE Yes
SPECIAL CARE INSTRUCTIONS See below.
Plastic impregnation, sometimes with dye, improves durability and color, common, stable. Detection, low SG, hot point, magnification.
Wax impregnation, improves color, common, may pick up dirt and discolor, detect with hot point, magnification.
Epoxy or CA impregnation, improves color slightly, makes porous material stronger and able to accept a polish, common, stable, detect with magnification.
Dying with shoe polish, enhances webbing, common, stable except to acetone, detect by wiping with acetone.
Epoxy backing, adds strength and weight, common, may separate, detected by sight.
Surface coating with epoxy, lacquer, etc. Improves color, seals dye, rare, stability varies, detect with magnification.
For more details see the article on "Hardness and Wearability."
As early as 4000 - 5000 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.
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.
Long before the arrival of Europeans in North American, the native inhabitants had an appreciation for turquoise, which is still strong today. The significance of this gem for some tribal groups rivals the importance that the ancient Chinese gave to jade. Such was also true in South America and the early Spanish invaders recorded their surprise in finding turquoise to be more highly valued by the native people than was gold.
During the 1960's and 70's there was a burst of admiration for turquoise among the US population, especially as found in Native American jewelry. After reaching the heights for a few years, this fad crashed amidst scandals involving simulants, imitations and "Indian" jewelry, (even that being sold by some Native Americans,) which was made in Asia.
The popularity of this gem is now once again at stratospheric levels, due to a combination of some well known modern designers favoring the stone and aggressive promotion by home shopping channels and fashion magazines. Although still widely available in traditional silver Southwestern style pieces, more and more designers are emulating the ancient Persians and the Victorians and setting pieces in gold.
The highest grades of turquoise are used for cabochons, carvings, and inlay, with the lesser grades finding use as polished beads or natural "nugget-style" beads.
Among those who prefer the look of turquoise with visible matrix, the highest regard is generally given to material with an even, interconnected patterning of black matrix veins. Stones of this type are referred to a "spiderweb" turquoise.
A relatively new electro-chemical enhancement called the "Zachary Process" has recently been promoted as an alternative to traditional stabilization, and is said to increase both durability and evenness of color. Some large retailers are now marketing gems treated in this way under proprietary brand names.
Turquoise itself is infrequently dyed, but the white and grey-veined mineral, Howlite, accepts dye readily and is commonly found for sale. Unfortunately not all the dyed Howlite offered is properly labeled as "faux" turquoise. Howlite is sometimes sold in its natural state with the misnomer "white turquoise," but make no mistake about it, there is NO such thing as white turquoise!
In addition, there are numerous non-mineral imitations such as plastic, ceramic, and glass, offered with or without matrix. Synthetic turquoise is also available, the most well known type of which was first produced by the Gilson company in 1972.
A recent import from China, which is being sold as "yellow turquoise," has been featured at some shows and by some retailers. The natural color is actually a light yellow-green, but buyer beware, as some very bright sunshine or butter yellow dyed pieces are being offered without much effort to discriminate them from the unenchanced material.
About the only natural gem with which turquoise is likely to be confused is variscite which can look similar to green turquoise. Variscite and turquoise, in fact, sometimes occur together in a rock which has been dubbed "variquoise" which brings a premium price for its attractive patterns and combinations of colors.
Text and photos courtesy of Barbara Smigel at Artistic Colored Stones.
