|Crystallography||Triclynic, although usually massive|
|Refractive Index||1.590 - 1.650|
|Hardness (?)||5 to 6|
|Specific Gravity (?)||2.40 - 2.90|
|Density||Crystals 2.84; massive in the range 2.6-2.9. Iran, 2.75-2.85; United States. 2.6-2.7; China, 2.70; Eilat, Israel, 2.56-2.70; Sinai Peninsula, 2.81; Tibet, 2.72; Bahia, Brazil, 2.40-2.65.|
|Wearability (?)||* Poor|
|Enhancements||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.|
|Formula||CuAl6(PO4)4(OH)8. 5H20 + Fe|
OPTICS: Massive material gives shadow edge (mean refractive index) of 1.62.
Crystals: a = 1.61; β=1.62: γ= 1.65.
Biaxial (+), 2V = 40°.
SPECIAL CARE INSTRUCTIONS See below.
OCCURRENCE: Turquoise is formed by the action of percolating groundwaters in aluminous rocks where Cu is present, as in the vicinity of copper deposits.
Lynch Station, Virginia: the only well known occurrence of crystals. These are microscopic, but an occasional larger one could be tempting to a cutter, and some very tiny faceted gems might exist (well under 1 carat)
lran: the district of Nishapur, on Ali-mersai Mountain.
Turquoise is found in porphyry and trachyte rocks, cemented by brown limonite. The color is uniform a lovely sky blue, sometimes veined by thin lines of limonitic matrix. The blue is often very intense. The mines have been worked for centuries, and Persian (now Iranian) turquoise is almost synonymous with material of the highest quality.
Tibet: turquoise is the national gem of this country, and green is the most prized color, very little material is available today.
China: some mines appear to have operated there in ancient times. Archaeological finds dated as early as 1300 B.C. indicate the possibility of a centuries-old exploitation of local deposits. Fine turquoise is currently mined in the Wudang mountain area of northwestern Hubei Province and also in Shaanxi Province about 150 km to the northwest. The material occurs as compact nodules typically up to 8 cm. with much larger masses occasionally found. The color ranges from pale blue to light green.
Egypt: on the Sinai Peninsula, turquoise is mined at Serâbît el Khâdim and Maharâh. These mines operated as early as 1000 B.C., and the turquoise was used by the Pharaohs. The producing area extends along the Suez Gulf, where the material occurs in sandstone. Earth
movements have brecciated the turquoise and matrix, and there is considerable limonite present. The color is blue to greenish blue; some may fade in the sun.
USSR: turquoise is reported from the Uzbek Republic, Chile: at the Chuquicamata copper mine turquoise of very fine color is found. Not much has reached the marketplace.
Australia: dense, compact turquoise of fine color has been found in large deposits. This material is solid, takes a high polish, and is uniform in color. The nodules in which it occurs may reach a size of hundreds of pounds.
The material has a slight tendency to shear along planes of weakness. The color resembles that of Persian (Iranian) turquoise.
Mexico: some turquoise has been reported from Zacatecar. Pau a Pique, Bahia, Brazil: Porous and cryptocrystalline material, R.I. ~ 1.618.
United States: there are many turquoise localities in the United States. Connoisseurs can tell the actual mine of origin of many cut gemstones because of distinctive nuances in color and matrix. The variation in these characteristics is enormous. Most of the mines are in Nevada, some are in Arizona, and others are in Colorado and New Mexico. Among the better known localities are:
Fox Mine (Nevada): huge production; active since 1915.
Blue Gem Mine (Nevada): large variation in color, noted for blue and green colors in the same stone.
Stormy Mountain Mine (Nevada): dark blue, hard material with black chert matrix.
Lander Blue Mine (Nevada): finely divided spiderweb, with tiny turquoise specks: this is rare and highly valued today.
Bisbee (Arizona): intense dark blue material, wispy matrix.
Kingman (Arizona): some deep blue material has been treated to improve color.
Leadville (Colorado): small stones, deep blue with a tinge of green.
Santa Rita (New Mexico): pale to deep blue colors.
Other notable locations in Nevada are the following mines: Papoose, Zuni, Montezuma, Crow Springs, Carlin, Red Mountain, Godber.
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.
[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.
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.
Simulants and Enhancements
There are numerous enhancements and simulants in the marketplace. All but the highest grades of turquoise may be “stabilized” by a pressure infusion of wax or epoxy resin. Small, porous pieces are sometimes pressed together with a resin binder to make a stabilized mosaic. Although it is usually true that Chinese turquoise has softer matrix, and tends to be more porous than that from much of the American Southwest, the photos below show, stabilization or lack of it is not always obvious and it’s dangerous to generalize.
Closeup of stabilized mosaic slab showing areas of different colors.
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!
A nugget of dyed Howlite sawn open to reveal its true nature.
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.
Care and Use
As a gem material turquoise has its limitations: it is relatively fragile, porous, and susceptible to heat and/or chemical damage. Turquoise averages 18 – 20% water content and, as the gem is heated, (perhaps from an unwary jeweler’s torch,) that water is progressively lost until at 400 degrees C, the structural integrity of the mineral is destroyed.
Few deposits of material are of such a fine grained and compact nature that they take a good polish. For these reasons, the majority of turquoise found in commerce today has been enhanced in one way or another. Even top grade, otherwise natural, stones are often given a surface coat of paraffin wax to seal them and enhance the polish. Skin oils and cosmetic residues are prime culprits in changing and darkening the color of turquoise gems.
Due to this stone’s properties, it is best to make turquoise rings and bracelets occasional wear items, and to protect all turquoise jewelry from heat, chemicals and shocks. So, no ultrasonic or steam cleaning, and wash only with mild, lukewarm soapy water and a soft brush, and wipe pieces with a damp cloth after wearing.
There is an avid collector market for turquoise, with sibling rivalry amongst the various enthusiasts who see virtue in different colors, matrix variations and mine sites. Just as no gem collection would be complete without several representatives of this species, so no jewelry collection should be without at least one piece featuring this well beloved December birthstone gem.
Evenness and saturation of color are the most important considerations in terms of value, followed closely by the degree to which the material is compact and capable of taking a good polish without stabilization. Among those who appreciate matrix patterns, the beauty of that pattern would be crucial in setting value. In my opinion, turquoise is a real gem bargain, for even the very highest grades of material are modestly priced compared to many other gems.
Source/Attribution: Barbara Smigel of Artistic Colored Stones