Added over 5 years ago
Turquoise (chemical formula CuAl6(PO4)4(OH)8·4H2O) is formed when acid-laden water percolates through weathered rocks that contain copper, aluminium, phosphorus and other minerals. Climate factors also play a role in the formation of turquoise as it is typically found only in arid regions containing highly fractured and weathered volcanic rocks. The process of formation may take up to 30 million years to complete.
Turquoise nearly always forms by filling veins, fractures and spaces in the surrounding rock (it is described as "massive" by geologists ie assuming no definite shape). The mother rock is often incorporated into the turquoise stone, forming characteristic veins, splotches and networks of brown, yellow and black, which is called "matrix".
Turquoise may range in colour from palest to darkest of blues, through shades of green and even into yellow. The blue in turquoise is enhanced when more copper is present, however if the rock in which the turquoise is formed contains aluminium, green will be the prevailing tone. A yellow-green turquoise form when zinc and iron is present. The colour of turquoise can change even after mining as a result of heating or contact with chemicals.
On Mohr's scale of hardness, turquoise may rate as soft as 2 and reaches a maximum hardness of just under 6 (slightly greater than window glass). Despite its softness relative to other gemstones, turquoise can take a good polish, although even the best quality material is fractious.
The World's Deposits
Turquoise was amongst the first gemstones to be mined by human beings. The oldest known workings, now over 5000 years old, are in Persia (modern day Iran), and these are still considered the most significant source of the stone because of its perfection of colour, superior hardness and lack of matrix.
The ancient Egyptians used turquoise since at least the First Dynasty (3000 BCE). The source of their material was the Sinai Peninsula. The colour of this material is typically darker than that mined in Iran but is considered to be of a very superior quality. Sinai and Iranian turquoise was probably the source of the first material to ever reach Europe, and in fact the word turquoise was derived around the16th century from the French for "Turkish" (Turquois), in which country it was extensively traded from, and/or for the word for dark-blue stone (pierre turquin).
The Southwest United States is considered to be the next most significant source of turquoise - Arizona, California, Colorado, New Mexico and Nevada are particularly important. Pre-Columbian Native Americans mined the deposits of California and New Mexico (most of which are exhausted today), while the Nevada deposits were first worked by North American Indians. Fine turquoise rivalling Iranian material in colour and hardness can sometimes be found, however most American turquoise is of a "friable" grade with green and yellow colours predominating and a large proportion of matrix present (considered, however to be very attractive when in "spiderweb" form). Some of America's most famous mines are the Bisbee, Kingman Turquoise and Sleeping Beauty Mines in Arizona and the Carico Lake and Ithica Peak Mines in Nevada.
While turquoise is valued very highly by native Tibetans, China and Tibet were considered to be a relatively minor source of turquoise over the past 3,000 years. However this is changing. China now has four or five significant turquoise-producing regions, most of which yield a fairly high to medium quality stone for export. Some Tibetan deposits are considered to be of the finest quality, hard and lustrous when polished with a dark, consistent matrix and a strongly greenish tint. However, the Chinese are currently flooding the market with imitations, fakes and reconstituted or pressed substitutes. As a result, many dealers will not buy Chinese turquoise unless they are absolutely sure of its source.
Other minor localities for turquoise deposits in the world are Afghanistan, Australia, northern Chile, Cornwall, Saxony, Silesia and Turkmenistan.
History of Use
Turquoise has been loved and valued by many of the world's great ancient civilizations, including that of the Egyptians and Aztecs and the Indus Valley, Persian and Mesopotamian nations, and later the great Muslim and Mogul empires. A common belief shared by many of these people was that turquoise possessed the ability to protect the wearer from harm and could cure most ailments, including blindness, bone disorders, bites and stings and even insanity. Turquoise was also regarded as a symbol of prosperity in many cultures, becoming a major trade and barter item all over the ancient civilized world. Tibetans wore the stone in their hair and used it for currency, while American Indians traded stones between tribes and later fashioned it into silver and beaded jewellery. Today in many parts of the world the stone is still regarded as a harbinger of good fortune, success and health, and is universally prized as an ornamental stone of great beauty.
In Western culture, turquoise is also the traditional birthstone for those born in the month of December.
Turquoise was so loved by the ancient Egyptians that it became (arguably) the first gemstone to be imitated via the glazed ceramic known as faience. In recent times the demand for turquoise has created a huge industry related to the synthesising of stones, while treatments for natural stones abound. Some of these treatments are difficult to detect even by experts.
The most common imitation of turquoise encountered today is dyed howlite, a stone similar in appearance and weight to turquoise and containing natural black veining, but white in its natural state. Magnesite is also sometimes substituted, although it lacks the black matrix that makes dyed howlite so convincing. It is very difficult for the average person to tell the difference between real turquoise and dyed howlite, as tests require at the very least examination under magnification or the application of hydrochloric acid.
Waxing and Oiling
Turquoise is an inherently fragile stone that varies greatly in colour and durability, and may therefore be treated to enhance both properties. Once again there is historical precedent for treating turquoise stones. In both ancient and relatively modern times waxes and oils were used to produce a wetting effect, simultaneously enhancing the natural colour and lustre of the stone and increasing its hardness. This method of treatment is now considered reasonably obsolete as oiled and waxed stones may "sweat" even under gentle heat or if exposed to too much sun, or the wax may rub off, creating unsightly and uneven patches of dullness. However it is accepted by tradition, especially because the turquoise so treated must be of a high grade to begin with.
Today turquoise is most commonly treated with epoxy resins, plastics or water glass in a process of impregnation termed "stabilization" or "bonding". This may occur by dipping the stone briefly or via pressure. By this means otherwise unsaleable turquoise has become available to the general gemstone market. Like waxing and oiling, stabilization creates a wetting effect and improves the durability of the stone, however it is a far more permanent and stable method of enhancement.
Stabilization is rejected by a few in the industry as too radical an alteration to the natural stone. However, it should be remembered that without such treatment most mines operating today would be unprofitable, the price of turquoise would increase astronomically, and the amount of turquoise available on the market would decrease by almost 90%, giving rise to a whole new industry for fake stones and reducing availability to those who love it.
As a buyer of turquoise jewellery, one should assume that almost all stones available for purchase today have had a treatment of wax, oil or plastic resins applied. The only means of certainty however are to test the stone with a heated probe or drill bit, which should give rise to the smell of plastic or oil
Dye may be used in conjunction with bonding treatments to create a more desirable shade of blue. In some cases traditional dyes such as Prussian Blue are used, however Chinese turquoise is now commonly dyed with Harpic toilet blue. This enhancement process is generally accepted within the industry but may be considered fraudulent when the stone is sold as "natural turquoise". When combined with stabilization the colour is usually steady, however some dyes may rub off or fade. A stone broken in half will reveal whether it has been treated and to what extent. Since this is rarely practical when buying jewellery, the need to buy from a reputable dealer is paramount.
The increasing demand for turquoise jewellery has given rise to a radical new development, whereby powdered fragments of turquoise stone and/or other types of filler are pressed into the form of a solid stone, down to the addition of convincing matrix. This treatment is misleadingly termed "reconstitution", since most, if not all, of the reconstituted material is likely to be artificial. When sold as an inexpensive turquoise substitute for the purpose of creating costume jewellery there can be no fault found with the product; however when reconstituted material is sold as the real thing, it becomes a more serious matter. China leads the world in creating reconstituted turquoise, some of which is passed off as natural turquoise via the Tibetan and Nepalese markets or inserted into wholesale shipments of real turquoise in order to pad out large orders.
In general, when buying turquoise jewellery, as a customer the rule of thumb is to buy from a reputable dealer or jeweller, who in turn will buy from mines, stone cutters or stone wholesalers that they trust. If a deal looks too good to be true - it is. No exceptions. Remember that natural or lightly treated turquoise is expensive to buy and will cost more than dyed or reconstituted material. However if price is important, there are many beautiful substitutes available at a fraction of the cost of the real thing.
Care and Maintenance
As a porous and fragile stone, turquoise is sensitive to knocks, chemicals and heat - although it should be noted that modern stabilization processes provide stones with considerable protection. Perfume and cosmetics may damage the finish of stones as will commercial jewellery cleaners, while skin oils or exposure to sunlight may alter turquoise colour. As a general rule turquoise jewellery should not be worn to the beach or in water, and all cosmetics should be applied before putting on jewellery. After wear, clean turquoise with a soft cloth and store separately to avoid scratching.
References and Further Reading
Hurlbut, CS, Klein, C, 1985. Manual of Mineralogy, 20th ed., John Wiley and Sons, New York ISBN 0-471-80580-7
Martin Watson, June 1983. Turquoise - The Gemstone of Tibet. The Tibetan Review http://www.dharamsalanet.com/links/articles/turquoise.htm
Pogue, J. E., 1915. The turquoise: a study of its history, mineralogy, geology, ethnology, archaeology, mythology, folklore, and technology. National Academy of Sciences, The Rio Grande Press, Glorieta, New Mexico. ISBN 0-87380-056-7
Turquoise, 2009. Wikipedia http://en.wikipedia.org/wiki/Turquoise