Turquoise

This article is about the gem. For the color, see cyan.

Turquoise

General
Category	Mineral
Formula	CuAl₆(PO₄)₄(OH₈)₈·5H₂O
Identification
Colour	Blue, blue-green, green
Habit	Massive, nodular, drusy
System	Triclinic
Cleavage	Good to perfect - usually N/A
Fracture	Conchoidal
Hardness	5-6
Lustre	Waxy to subvitreous
RI	1.61-1.65 (DR +0.040)
Pleochroism	N/A
Streak	Bluish white
SG	2.6-2.9
Fusibility	Fusible in heated HCl
Solubility	Soluble in HCl

Turquoise (or turquois) is an opaque, blue to green hydrated copper aluminium phosphate mineral according to the chemical formula CuAl₆(PO₄)₄(OH₈)₈·5H₂O. It is rare and valuable in finer grades, and has been enjoyed as a gem and ornamental stone for untold thousands of years, owing to its unique colour. In recent times turquoise—like most other opaque gems—has had its popularity undermined by the introduction of myriad treatments, imitations, and so-called synthetics onto the market, some difficult to detect even by experts.

The word turquoise is very old and of uncertain origin, derived from the French pierre turquoise, meaning "Turkish stone". This is thought to have arisen from a misconception: turquoise does not occur in Turkey but was traded there, and the gem thus became associated with the country in the West.

Properties

Even the finest of turquoise is rather fragile, reaching a maximum hardness of just under 6, or slightly less than window glass. Characteristically a cryptocrystalline mineral, turquoise almost never forms single crystals and all of its properties are highly variable. Its crystal system is proven to be triclinic via X-ray diffraction testing. With lower hardness comes lower specific gravity (high 2.90, low 2.60) and greater porosity: These properties are dependent on grain size. The lustre of turquoise is typically waxy to subvitreous, and transparency is usually opaque, but may be semitranslucent in thin sections. Colour is as variable as the mineral's other properties, ranging from white to a powder blue to a sky blue, and from a blue-green to a yellowish green. The blue is attributed to idiochromatic copper while the green may be the result of either iron impurities (replacing aluminium) or dehydration.

The refractive index (as measured by sodium light, 589.3 nm) of turquoise is approximately 1.61 or 1.62; this is a mean value seen as a single reading on a gemological refractometer, owing to the almost invariably polycrystalline nature of turquoise. A reading of 1.61-1.65 (birefringence 0.040, biaxial positive) has been taken from rare single crystals. An absorption spectrum may also be obtained with a hand-held spectroscope, revealing a line at 432 nanometres and a weak band at 460 nanometres (this is best seen with strong reflected light). Under longwave ultraviolet light, turquoise may occasionally fluoresce green, yellow or bright blue; it is inert under shortwave ultraviolet and X-rays.

Turquoise is infusible in all but heated hydrochloric acid. Its streak is a pale bluish white and its fracture is conchoidal, leaving a waxy lustre. Despite its low hardness relative to other gems, turquoise takes a good polish. Turquoise may also be peppered with flecks of pyrite or interspersed with dark, spidery limonite veining.

Formation

As a secondary mineral, turquoise forms by the action of percolating acidic aqueous solutions during the weathering and oxidation of pre-existing minerals; for example, the copper may come from malachite, azurite, or sulfides; the alumina from feldspar, and the phosphorus from apatite. Atmospheric factors may play an important role: Turquoise is typically found in arid regions, filling or encrusting cavities and fractures in sandstone or volcanic rocks, often with associated limonite and other iron oxides. There are two hypotheses invoked to explain the precise mechanisms involved in the formation of turquoise:

The hypogene hypothesis, which holds that the aqueous solutions originate at great depth, from hydrothermal processes. Initially at high temperature, these solutions rise upward to surface layers, interacting with and leaching essential elements from pre-existing minerals in the process. As the solutions cool, turquoise precipitates, lining cavities and fractures within the surrounding rock.
The supergene hypothesis, which holds that the aqueous solutions are meteoric in origin; that is, arising from rain water seeping down through surface rocks. The turquoise thus precipitates from a cool solution which weathered existing minerals over a much longer time frame than is supposed by the hypogene hypothesis.

Turquoise is nearly always cryptocrystalline and therefore assumes no definite external shape (massive); these deposits may be nodular or botryoidal in habit. Turquoise may also pseudomorphously replace feldspar or fossils, such as clam shells. Intergrowth with other secondary copper minerals such as chrysocolla is also common.

Occurrence

Turquoise was among the first gems to be mined, and while many historic sites have been depleted, some are still worked to this day. These are all small-scale, often seasonal operations, owing to the limited scope and remoteness of the deposits. Most are worked by hand with little or no mechanization. However, turquoise is often recovered as a byproduct of large-scale copper mining operations, especially in the United States.

Iran

For at least 2,000 years, the region once known as Persia has remained the most important source of turquoise, for it is here that fine material is most consistently recovered. This "perfect colour" deposit is restricted to a mine-riddled 2,012 metre mountain peak of Ali-mersai, 25 km from Mashhad, the capital of Khorasan province, Iran. A weathered and broken trachyte is host to the turquoise, which is found both in situ between layers of limonite and sandstone, and amongst the scree at the mountain's base. These workings, together with those of the Sinai Peninsula, are the oldest known.

Iranian turquoise is often found replacing feldspar. Although it is commonly marred by whitish patches, its colour and hardness are considered superior to the production of other localities. Iranian turquoise has been mined and traded abroad for centuries, and was probably the source of the first material to reach Europe.

Sinai

Since at least the First Dynasty (3,000 BCE), and possibly before then, turquoise was used by the Egyptians and was mined by them in the Sinai Peninsula, called "Country of Turquoise" by the native Monitu. There are six mines in the region, all on the southwest coast of the peninsula, covering an area of some 650 km². The two most important of these mines, from a historic perspective, are Serabit el-Khadem and Wadi Maghareh, believed to be among the oldest of known mines. The former mine is situated about 4 kilometres from an ancient temple dedicated to Hathor.

The turquoise is found in sandstone; large-scale exploitation is not profitable today, but the deposits are sporadically quarried by Bedouin peoples using homemade gunpowder. In the rainy winter months, miners face a risk from flash flooding; even in the dry season, death from the collapse of the halphazardly exploited sandstone mine walls is not unheard of. The colour of Sinai material is typically greener than Iranian material, but is thought to be stable and fairly durable. Often referred to as Egyptian turquoise, Sinai material is typically the most translucent, and under magnification its surface structure is revealed to be peppered with dark blue discs not seen in material from other localities.

In proximity to nearby Eilat, Israel, an attractive intergrowth of turquoise, malachite, and chrysocolla is found. This rock is called Eilat stone and is often referred to as Israel's national stone: it is worked by local artisans for sale to tourists.

United States

The Southwest United States is a significant source of turquoise; Arizona, California (San Bernardino, Imperial, and Inyo counties), Colorado (Conejos, El Paso, Lake, and Saguache counties), New Mexico (Eddy, Grant, Otero, and Santa Fe counties) and Nevada are or were especially rich. The deposits of California and New Mexico were mined by pre-Columbian Native Americans using stone tools, some local and some from as far away as central Mexico. Cerrillos, New Mexico is thought to be the location of the oldest mines; prior to the 1920s, the state was the country's largest producer; it is more or less exhausted today. Only one mine in California, located at Apache Canyon, operates at a commercial capacity today.

The turquoise occurs as vein or seam fillings, and as compact nuggets; these are mostly small in size. While quite fine material—rivalling Iranian material in both colour and durability—is sometimes found, most American turquoise is of a low grade (called "chalk turquoise"); high iron levels mean greens and yellows predominate, and a typically friable consistency precludes use in jewellery in the turquoise's untreated state. Arizona is currently the most important producer of turquoise by value, with the vivid Bisbee Blue being a good example of the state's natural endowment; much of the Arizona material is recovered as a byproduct of copper mining. Nevada is the country's other major producer, with an estimated 75–100 mines opened over the state's history. The Nevada material is noted for its often attractive brown or black limonite veining, producing what is called "spiderweb matrix".

In 1912, the first deposit of distinct, single-crystal turquoise was discovered in Lynch Station, Campbell County, Virginia. The crystals, forming a druse over the mother rock, are very small; 1 mm (0.04 inches) is considered large. Until the 1980s Virginia was widely thought to be the only source of distinct crystals; there are now at least 27 other localities.[1] The specimens are highly valued by collectors.

In an attempt to recoup profits and meet demand, most American turquoise is treated or enhanced to a certain degree. These treatments include innocuous waxing and more controversial procedures, such as dyeing and impregnation (see Treatments).

Other sources

China has been a minor source of turquoise for 3,000 years or more. Gem-quality material, in the form of compact nodules, is found in the fractured silicified limestone of Yunxian and Zhushan, Hubei province. Additionally, Marco Polo reported turquoise found in present-day Sichuan. Most Chinese material is exported, but a few carvings worked in a manner similar to jade exist. In Tibet, where green turquoise has long been appreciated, gem-quality deposits purportedly exist in the mountains of Derge and Nagari-Khorsum in the east and west of the region respectively. However, the existence of these deposits is doubted by some due to a lack of corroboration.

Other notable localities include Afghanistan, Australia (Victoria and Queensland), northern Chile (Chuquicamata), Cornwall, Saxony, Silesia, and Turkestan.

History of use

The pastel shades of turquoise have endeared it to many great cultures of antiquity: it has adorned the rulers of Ancient Egypt, the Aztecs (and possibly other Pre-Columbian Mesoamericans), Persia, Mesopotamia, the Indus Valley, and to some extent in ancient China since at least the Shang Dynasty [2]. Despite being one of the oldest gems, probably first introduced to Europe (through Turkey) with other Silk Road novelties, turquoise did not become important as an ornamental stone in the West until the 14th century, following a decline in the Catholic Church's influence which allowed the use of turquoise in secular jewellery. It was apparently unknown in India until the Muhgal period, and unknown in Japan until the 18th century. A common belief shared by many of these civilisations held that turquoise possessed certain prophylactic qualities; it was thought to change colour with the wearer's health and protect him or her from untoward forces.

The Aztecs inlaid turquoise, together with gold, quartz, malachite, jet, jade, coral, and shell, into provocative (and presumably ceremonial) mosaic objects such as masks (some with a human skull as their base), knives, and shields. Natural resins, bitumen and wax were used to bond the turquoise to the objects' base material; this was usually wood, but bone and shell were also used. Like the Aztecs, the Pueblo, Navajo and Apache tribes cherished turquoise for its amuletic use; the latter tribe believe the stone to afford the archer dead aim. Among these peoples turquoise was used in mosaic inlay, in sculptural works, and was fashioned into toroidal beads and freeform pendants. The Ancestral Puebloans (Anasazi) of the Chaco Canyon and surrounding region are believed to have prospered greatly from their production and trading of turquoise objects. The distinctive silver jewellery produced by the Navajo and other Southwestern Native American tribes today is a rather modern development, thought to date from circa 1880 as a result of European influences.

In Persia, turquoise was the de facto national stone for millennia, extensively used to decorate objects (from turbans to bridles), mosques, and other important buildings both inside and out, such as the Medresseh-I Shah Husein Mosque of Isfahan. The Persian style and use of turquoise was later brought to India following the establishment of the Mughal Empire there, its influence seen in high-karat gold jewellery (together with ruby and diamond) and in such buildings as the Taj Mahal. Persian turquoise was often engraved with devotional words in Arabic script which was then inlaid with gold.

File:Tutankhamun-mask.jpg

The iconic gold burial mask of Tutankhamun, inlaid with turquoise, lapis lazuli, carnelian and coloured glass.

Cabochons of imported turquoise, along with coral, was and still is used extensively in the silver and gold jewellery of Tibet and Mongolia, where a greener hue is said to be preferred. Most of the pieces made today, with turquoise usually roughly polished into irregular cabochons set simply in silver, are meant for inexpensive export to Western markets and are probably not accurate representations of the original style.

The Egyptian use of turquoise stretches back as far as the First Dynasty and possibly earlier; however, probably the most well-known pieces incorporating the gem are those recovered from Tutankhamun's tomb, most notably the Pharaoh's iconic burial mask which was liberally inlaid with the stone. It also adorned rings and great sweeping necklaces called pectorals. Set in gold, the gem was fashioned into beads, used as inlay, and often carved in a scarab motif, accompanied by carnelian, lapis lazuli, and in later pieces, coloured glass. Turquoise, associated with the goddess Hathor, was so liked by the Ancient Egyptians that it became [arguably] the first gemstone to be imitated, the fair semblance created by an artificial glazed ceramic product known as faience. (A similar ceramic has been recovered from Bronze Age burial sites in the British Isles.)

The mid 19th century and early 20th century archaeological excavations of Egypt by the French, including that of Tutankhamun's tomb, created a great public interest in the western world, subsequently influencing jewellery, architecture, and art of the time. Turquoise, already favoured for its pastel shades since c. 1810, was a staple of Egyptian Revival pieces. In contemporary Western use, turquoise is most often encountered cut en cabochon in silver rings, bracelets, often in the Native American style, or as tumbled or roughly hewn beads in chunky necklaces. Lesser material may be carved into fetishes, such as those crafted by the Zuni. While strong sky blues remain superior in value, mottled green and yellowish material is popular with artisans. In Western culture, turquoise is also the traditional birthstone for those born in the month of December.

Imitations

The Egyptians were apparently the first to produce an artifical imitation of turquoise, in the glazed earthenware product faience. Later glass and enamel were also used, and in modern times more sophisticated ceramics, porcelain, plastics, and various assembled, pressed, bonded, and sintered products (composed of various copper and aluminium compounds) have been developed: examples of the latter include "Viennese turquoise", made from precipitated aluminium phosphate coloured by copper oleate; and "neolith", a mixture of bayerite and copper phosphate. Most of these products differ markedly from natural turquoise in both physical and chemical properties, but in 1972 Pierre Gilson introduced one fairly close to a true synthetic. It does differ in chemical composition owing to a binder used, meaning it is best described as a simulant rather than a synthetic. Gilson turquoise is made in both a uniform colour and with black "spiderweb matrix" veining not unlike the natural Nevada material.

Some natural blue to blue-green materials, such as this botryoidal chrysocolla with quartz drusy, are occasionally confused with, or used to imitate turquoise.

The most common imitation of turquoise encountered today is dyed howlite and magnesite, both white in their natural states, and the former also having natural and convincing black veining similar to that of turquoise. Dyed chalcedony, jasper, and marble is less common, and much less convincing. Other natural materials occasionally confused with or used in lieu of turquoise include variscite, faustite, chrysocolla (especially when impregnating quartz), lazulite, smithsonite, hemimorphite, wardite, and a fossil bone or tooth called odontolite or "bone turquoise", coloured blue naturally by the mineral vivianite. While rarely encountered today, odontolite was once mined in large quantities—specifically for its use as a substitute for turquoise—in southern France.

These fakes are detected by gemologists using a number of tests, relying primarily on non-destructive, close examination of surface structure under magnification; a featureless, pale cyan background peppered by flecks or spots of whitish material is the typical surface appearance of natural turquoise, while manufactured imitations will appear radically different in both colour (usually a uniform dark blue) and texture (usually granular or sugary). Glass and plastic will have a much greater translucency, with bubbles or flow lines often visible just below the surface. Staining between grain boundaries may be visible in dyed imitations.

Some destructive tests may however be necessary; for example, the application of diluted hydrochloric acid will cause the carbonates odontolite and magnesite to effervesce and howlite to turn green, while a heated probe may give rise to the acrid smell so indicative of plastic. Differences in specific gravity, refractive index, light absorption (as evident in a material's absorption spectrum), and other physical and optical properties are also considered as means of separation. Imitation turquoise is so prevalent that it likely outnumbers real turquoise by a wide margin. Even material used in authentic Native American and Tibetan jewellery is often fake or, at best, heavily treated.

Treatments

Turquoise is treated to enhance both its colour and durability (increased hardness and decreased porosity). Historically, light waxing and oiling were the first treatments to be used (since ancient times), providing a wetting effect (thereby enhancing the colour and lustre); this treatment is more or less acceptable by tradition, and because such material is usually of a higher grade to begin with. Conversely, the later development of pressure impregnation of otherwise unsaleable chalky American material by epoxy and plastics (such as polystyrene) and water glass—also producing a wetting effect in addition to improving durability—are rejected by some as too radical an alteration. Plastic and water glass are technologically superior to oil and wax in that the former treatment are far more permanent and stable, and can be applied to material too friable for oil or wax to be of sufficient help; such material is termed "bonded" or "stabilized" turquoise. The epoxy binding technique was first developed in the 1950s and has been attributed to Colbaugh Processing of Arizona, a company that still operates today. The majority of American material is now treated in this manner. Although it is a costly process requiring many months to complete, without impregnation most American mining operations would be unprofitable.

Oiled and waxed stones are also prone to "sweating" under even gentle heat or if exposed to too much sun, and they may develop a white surface film or bloom over time. (With some skill, oil and wax treatments can be restored.) Likewise, the use of Prussian blue and other dyes—often in conjunction with bonding treatments—to enhance (make uniform or completely change) colour is regarded as fraudulent by purists—especially since some dyes may fade or rub off on the wearer. Dyes have also been used to darken the veins of turquoise. Perhaps the most radical of treatments is "reconstitution", wherein supposedly fragments of fine material too small to be used singly are powdered and then bonded to form a solid mass. Much if not all of this "reconstituted" material is likely a complete fabrication (with no natural components), or may have foreign filler material added to it (see Imitations section). Another treatment—the details of which remain undisclosed—is the so-called Zachery process, named after its developer, electrical engineer and turquoise trader James E. Zachery. This process claims to use only medium grade material at a minimum, leaving the turquoise harder and with a better colour and lustre.

As the finer turquoise is often found as thin seams, it may be glued to a base of stronger foreign material as a means of reinforcement. These are termed doublets and can be very deceptive in certain jewellery setting styles (such as closed back and bevel settings). Some turquoise is cut with the mother rock serving as a base; these are usually not considered doublets, but may have an intrinsic value lower than that of "whole" stones. Doublets, like the aforementioned treatments, are legal provided they are disclosed to the customer before sale.

As is so often with gems, full disclosure is frequently not given; it is therefore left to gemmologists to detect these treatments in suspect stones, using a variety of testing methods—some of which are necessarily destructive. For example, the use of a heated probe applied to an inconspicuous spot will reveal oil, wax, or plastic treatment with certainty.

Valuation and care

Richness of colour is the chief determiner of value in turquoise; generally speaking, the most desirable is a strong sky or "robin's egg". Value decreases with the increase of green hue, lightening of colour, and mottling. In Tibet however, a greener blue is said to be preferred. Whatever the colour, turquoise should not be excessively soft or chalky; even if treated, such lesser material (to which most turquoise belongs) is liable to fade or discolour over time and will not hold up to normal use in jewellery.

The mother rock or matrix in which turquoise is found can often be seen as splotches or a network of brown or black veins running through the stone in a netted pattern; this veining may add value to the stone if the result is complimentary, but such a result is uncommon. Such material is sometimes described as "spiderweb matrix"; it is most valued in the Southwest United States and Far East, but is not highly appreciated in the Near East where unblemished and vein-free material is ideal (regardless of how complimentary the veining may be). Uniformity of colour is desired, and in finished pieces the quality of workmanship is also a factor; this includes the quality of the polish and the symmetry of the stone. Calibrated stones—that is, stones adhering to standard jewellery setting measurements—may also be more sought after. Like coral and other opaque gems, turquoise is commonly sold at a price according to its physical size in millimetres rather than carat weight.

Turquoise is treated in many different ways, some more permanent and radical than others. Controversy exists as to whether some of these treatments should be acceptable, but one can be more or less forgiven universally: This is the light waxing or oiling applied to most gem turquoise to improve its colour and lustre; if the material is of high quality to begin with, very little of the wax or oil is absorbed and the turquoise therefore does not "rely" on this impermanent treatment for its beauty. All other factors being equal, untreated turquoise will always command a higher price. Bonded and "reconstituted" material is worth considerably less.

Being a phosphate mineral, turquoise is inherently fragile and sensitive to solvents; perfume and other cosmetics will attack the finish and may alter the colour of turquoise gems, as will skin oils. Prolonged exposure to direct sunlight may also discolour or dehydrate turquoise. Care should therefore be taken when wearing such jewels: cosmetics, including sunscreen and hairspray, should be applied before putting on turquoise jewellery, and they should not be worn to a beach or other sun-bathed environment. After use, turquoise should be gently cleaned with a soft cloth to avoid a build up of residue, and should be stored in its own box to avoid scratching by harder gems.

References and further reading

British Museum (2000). Aztec turquoise mosaics. Retrieved November 15, 2004 from www.thebritishmuseum.ac.uk
Dietrich, R. V. (2004). Turquoise. Retrieved November 20, 2004 from www.cst.cmich.edu/users/dietr1rv/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 0873800567
Schadt, H. (1996). Goldsmith's art: 5000 years of jewelry and hollowware. Arnoldsche Art Publisher, Stuttgard, New York. ISBN 3925369546
Schumann, W. (2000). Gemstones of the world, revised edition. Sterling Publishing. ISBN 0806994614
USGS (2002). Turquoise. An overview of production of specific U.S. gemstones. U.S. Bureau of Mines Special Publication 14-19. Retrieved November 15, 2004 from http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/turquoise.html
Webster, R. (2000). Gems: Their sources, descriptions and identification (5th ed.), pp. 254-263. Butterworth-Heinemann, Great Britain. ISBN 0750616741