Over the past half-century, turquoise has withstood the vicissitudes of fashion trends. Today, the striking sky blue to blue-green stone is favored by well-known modern jewelry designers as well as aficionados of American Southwestern and Native American jewelry. However, this is only a brief moment in the long history of turquoise gems. As early as 3,000 BCE, the Ancient Egyptians mined and worked turquoise into jewelry and ceremonial objects. Millennia later, the Chinese and Ancient American cultures did the same. Persia (Iran) introduced Medieval Europe to this stone. Turquoise continues to be highly desired and valued for jewelry and other decorative objects. It's also inspired many legends and folkloric beliefs. Although turquoise does have some physical limitations as a jewelry gem, with proper cutting, treatment, and care, turquoise jewelry can make a wonderful addition to your collection.
In terms of value, color evenness and saturation are the most important considerations, followed closely by the degree to which the material is compact and capable of taking a good polish without stabilization. Higher values in turquoise are generally associated with darker shades and less green tint in the blue color. For those who appreciate matrix patterns, their beauty would be crucial in setting value. Spiderweb turquoise, veined with black matrix in a pattern that looks like crocheted lace, is quite popular.
Turquoise enjoys an avid collector market, with sibling rivalries among 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, no jewelry collection should be without at least one piece featuring this beloved traditional December birthstone. (Turquoise is also the birthstone for those born on Saturday).
Turquoise is a real gem bargain. Even the very highest grades of material are modestly priced compared to many other gems.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Triclynic, although usually massive. Crystals extremely rare and microscopic; microcrystalline, massive; concretionary; veins and crusts.|
|Refractive Index||1.590 - 1.650|
|Colors||Crystals blue. Massive materials dark blue to pale blue, green, blue-green, apple green, grayish green.|
|Luster||Crystals vitreous; massive, waxy or dull, earthy.|
|Hardness||5 - 6|
|Specific Gravity||2.40 - 2.90. Crystals 2.84; massive in the range 2.6-2.9. See "Identifying Characteristics" below.|
|Cleavage||None in massive material.|
|Luminescence||Greenish yellow to blue in LW, inert in SW and X-rays.|
|Spectral||Can be distinctive; lines at 4600 (vague) and 4320—these are usually seen in light reflected from the turquoise surface.|
|Transparency||Translucent (rare), opaque|
|Formula||CuAl6(PO4)4(OH)8 · 5H2O + Fe|
|Pleochroism||Weak: colorless/pale blue or pale green.|
|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°.|
|Etymology||From the French turquois for “Turkish.” The stone was brought to Europe from Persia (Iran) through Turkey.|
|Occurrence||Formed by the action of percolating groundwaters in aluminous rocks where copper is present, as in the vicinity of copper deposits.|
Chemically, turquoise is a hydrated copper/aluminum phosphate of aggregate, cryptocrystalline structure. Only one deposit is known to produce transparent to translucent crystals: Lynch Station, Virginia. (Specimens from this locale are rare and bring a hefty price from collectors). More typically, this stone is found as an opaque deposit in nodules, in veins within host rocks, or as shallow crusts on the surface of rocks. Massive turquoise is always opaque.
Turquoise can form in series with the minerals chalcosiderite and planerite. Faustite has an intense yellow-green color and a density within the turquoise range. It’s the zinc analog of turquoise. Malachite and chrysocolla can sometimes grow together in stones with turquoise as well.
Colors range from shades of blue to blue-green to yellowish green depending on the amount of trace elements. Copper adds blue. Chromium and vanadium add green. 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. Historically, and even largely today, the most admired stones are those with a fine robin’s egg or celestial blue color, with no visible matrix. This shade indicates no iron and little vanadium is present. Sometimes referred to as “Persian grade,” turquoise of this sort is still produced in Iran. However, similar stones have been found in the US, particularly in the Sleeping Beauty Mine near Globe, Arizona.
Most rough contains patches or veins of the host rock in which it formed, such as chalcedony, opal, brown limonite, black chert, or white kaolinite. This brownish or black matrix is often cut along with the stone to provide color contrast and patterns. This matrix can affect the color, toughness, and workability of the stone. 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 porosity, while a high clay mineral content has the opposite effect. On one end of this spectrum, we find pieces of hardness 5.5 to 6 that take a bright polish and are minimally porous. On the other end, we have pieces of a soft and chalky nature so porous they’re unusable without stabilization.
Turquoise’s absorption spectrum can be distinctive. See the “Information Table” above.
The specific gravity of stones from different sources varies. While there is some overlap, some of the measurements at the extreme ends can help identify sources.
- Iran: 2.75-2.85
- United States: 2.6-2.7
- China: 2.70
- Eilat, Israel: 2.56-2.70
- Sinai Peninsula, Egypt: 2.81
- Tibet: 2.72
- Bahia, Brazil: 2.40-2.65
Synthetic turquoise is available, with or without matrix. The most well-known type was created by the Pierre Gilson Company in 1972. It can resemble the finest Persian grade stone, but a microscope will reveal the difference. Natural stones have a smooth surface. Under magnification, the synthetics display a mix of tiny blue spheres in a light-colored host medium that resembles the texture of “cream of wheat.”
There are many simulants on the market. Non-mineral imitations include plastics, ceramics, and glass and can appear very realistic. So-called “Viennese turquoise” is artificially blue-tinted argillaceous earth.
Some natural gemstones can be mistaken for turquoise. Variscite can look like green turquoise. In fact, variscites and turquoises sometimes occur together in rocks, which are dubbed “variquoise.” This attractive combination of patterns and colors can command a premium price. Proposite, especially the Mexican material with its light-blue color, is another potential simulant.
There are numerous enhancements for this stone. They’re very difficult to detect without detailed knowledge and the right testing equipment. Pale specimens are extensively treated to improve their color. Fine grained and compact material that will take a good polish is rare. Skin oils and cosmetic residues can easily darken the color of turquoise gems. For these reasons, most stones on the market have 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. 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. However, whether a stone has been stabilized is not always obvious, as the following photo illustrates.
An electro-chemical proprietary enhancement process called the “Zachery Treatment” has been promoted as an alternative to traditional stabilization that improves both durability and evenness of color.
Turquoise itself is not dyed frequently. However, howlite, a white and grey-veined mineral, readily accepts dye. Blue-dyed howlites are often found for sale. Unfortunately, they’re not always labeled as “faux turquoises.” Sometimes, howlite in its natural (albeit unremarkable) state is sold under the misnomer “white turquoise.” Buyer beware. There’s no such thing.
Dyed magnesite can also resemble turquoise. In recent years, this material has gained popularity as a simulant. If properly disclosed, this is acceptable. If not, beware beware, again.
Beware of “yellow turquoise” imported from China. There are stones that do have a natural light yellow-green color. However, some very bright sunshine or butter-yellow dyed pieces are being offered without much effort to discriminate them from the non-dyed material.
- Plastic impregnation, sometimes with dye: improves durability and color, common, stable. Detection, low specific gravity, hot point, magnification.
- Wax impregnation: improves color, common, may pick up dirt and discolor, detect with hot point, magnification.
- Epoxy 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.
Please note: hot point and dye testing are destructive tests. They should only be conducted as a last resort.
Turquoise usually occurs in arid regions, where ground water percolates through aluminous rock in the vicinity of copper deposits. Like azurite, malachite, and opal, it’s a secondary mineral that forms through the interaction of pre-existing minerals and their solutions. The majority of today’s commerce in turquoise is primarily from North America and China.
The US deposits are almost exclusively limited to the Southwest (with one notable exception). Nevada is home to more mines than Arizona, California, Colorado, and New Mexico put together. (Turquoise is the official state gemstone of Arizona, Nevada, and New Mexico). 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.
Among the better known localities are:
- 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).
- 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, and Godber.
Historically the finest material was obtained from mines in Persia (Iran). There is still considerable production from that area. Persian, now Iranian, turquoise is almost synonymous with material of the highest quality.
- District of Nishapur, on Ali-mersai Mountain: Found in porphyry and trachyte rocks, cemented by brown limonite. The color is a uniform lovely sky blue, sometimes veined by thin lines of limonitic matrix. The blue is often very intense. The mines have been worked for centuries.
Other Notable Sources
- 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 in ancient times. Archaeological finds dated as early as 1,300 BCE indicate the possibility of a centuries-old exploitation of local deposits. Fine material 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. Generally, Chinese stones have softer matrix and are more porous than the material from the American Southwest.
- Egypt: in the Sinai Peninsula, turquoise is mined at Serâbît el Khâdim and Maharâh. These mines operated as early as 1,000 BCE, and the material was used by the Pharaohs. The producing area extends along the Suez Gulf, where the material occurs in sandstone. Earth movements have brecciated the gem and matrix. There is considerable limonite present. The color is blue to greenish blue. Some may fade in the sun.
- Chile: at the Chuquicamata copper mine, material of very fine color is found. Not much has reached the marketplace.
- Australia: dense, compact material of fine color has been found in large deposits. This material is solid, takes a high polish, and is uniform in color. Found in nodules that may reach 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 material has been reported from Zacatecas.
- Pau a Pique, Bahia, Brazil: porous and cryptocrystalline material, RI ~ 1.618.
- Afghanistan; Kenya; Uzbekistan
Turquoise is relatively fragile, porous, and susceptible to heat and/or chemical damage. Stones average 18-20% water content. When heated, say from an unwary jeweler’s torch, that water is progressively lost until, at 400º C, the structural integrity of the mineral is destroyed. Due to this stone’s properties, turquoise jewelry is best reserved for occasional wear. Protect it from heat, chemicals, and shocks. Don’t use mechanical cleaning methods, such as ultrasonic or steam cleaning. Wash only with mild, lukewarm soapy water and a soft brush. Wipe pieces with a damp cloth after wearing.
Although the practice of setting turquoise in silver has a long tradition in the US, the stone is traditionally set in gold, sometimes with diamonds, in the Middle East. This is a Persian tradition as well. In the Victorian Era, turquoise was greatly admired and also typically set in gold. Today, more jewelry designers are emulating the ancient Persians and the Victorians and setting pieces in gold.