Turquoise

• 1. Introduction

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  Turquoise is an opaque mineral that runs from blue to green, and at Bisbee, Arizona, some of it once left a copper mine in the lunch pails of the miners. No contractor was ever hired to dig it. The workers carried it out themselves, alongside their sandwiches. That odd detail says something about a stone that has been prized for millennia and still turns up in unexpected places. Why does a gem named for Turkey not come from Turkey? How can rain and copper build something people set in gold and silver? And why is so much of what is sold as turquoise either a different material or a heavily altered one?

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• 2. A Borrowed Name

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  The year 1573 is the earliest recorded moment when English speakers used turquoise as the name of a color. The word for the stone dates to the 16th century and comes from the Old French turquois, meaning Turkish. The reason is the route the stone traveled. It reached Europe through the Ottoman Empire, but it was mined in the historical Khorasan province of Iran, then known as Persia. Turkey has no deposits of its own, so the name is a misnomer.

  Pliny the Elder called the stone callais, from an Ancient Greek word. The Aztecs knew it as chalchihuitl. The oldest Iranian name carried a more triumphant meaning. Iranians first called the gem pērōzah, which means victory, and the Arabs later reshaped it into fayrūzah. In Modern Persian the word is pronounced fīrūzeh.

  Mineralogists once had labels of their own. Until the middle of the 19th century, professional texts also used kalaite and azure spar, names that hinted at where the mineral came from. Neither caught on widely, and both slowly faded. Behind the Turkish label lay the mountains of Khorasan, and the rock there was shaped by a slow chemistry that begins with copper.

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• 3. Rain on Copper

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  Deep in copper country, hot watery fluids leach the metal out of a host rock. That rock is typically an oxidized intrusion rich in silica. The copper settles again in veins of copper sulfide that fill the joints and cracks, forming what geologists call a copper porphyry. Turquoise is not part of that original rock. It is a secondary mineral, often called supergene, and it arrives later.

  Rain or melting snow soaks down through the porphyry. Oxygen dissolved in the water turns the copper sulfides into soluble sulfates. The acidic, copper-rich liquid then reacts with aluminum and potassium minerals in the surrounding rock, and turquoise precipitates out. It fills veins in volcanic rock or in phosphate-rich sediments. This happens at fairly low temperatures and seems to happen more readily in arid places.

  The Sinai Peninsula shows one version of the process. Its turquoise sits in lower Carboniferous sandstones, capped by basalt flows and upper Carboniferous limestone. The overlying beds probably supplied the copper. The classic Iranian deposits formed in Tertiary sandstones and limestones that were invaded by apatite-rich trachytes. Weathering converted some minerals to alunite, freeing aluminum and phosphate to combine with copper. All this happened close to the surface. By 1965 the Iranian mines had bottomed out at an average depth of just 9 m. In Cochise County, Arizona, the deposits sit in Cambrian quartzites and young granites and reach at least 54 m.

  The result is a soft and variable stone. The finest turquoise reaches a Mohs hardness of just under 6, slightly more than window glass. It is cryptocrystalline, meaning its crystals are too small to see, and X-ray diffraction shows a triclinic structure. Lower hardness goes with greater porosity. The lustre is waxy to subvitreous, and the stone is opaque unless sliced thin. Color ranges from white through powder blue and sky blue to blue-green and yellowish green. Copper accounts for the blue, while iron impurities replacing copper may cause the green. Despite its softness, turquoise takes a good polish, and it may carry flecks of pyrite.

  Turquoise usually forms veins, nodules, or rounded botryoidal masses with no fixed outer shape. It can replace feldspar, apatite, other minerals, and even fossils. Only its greater hardness separates it from chrysocolla, the one common mineral with similar traits. It also forms a complete series with chalcosiderite, in which ferric iron stands in for aluminium. Real single crystals are so scarce that the first deposit of distinct ones, found in 1912 at Lynch Station in Campbell County, Virginia, made news. Those crystals form a druse over the host rock, and 1 mm counts as large. Until the 1980s Virginia was widely thought to be the only source, but at least 27 other localities are now known.

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• 4. Persia and Sinai

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  The peak of Ali-mersai, 2012 m high and near Mashhad, the capital of Khorasan Province, sits at the heart of the classic Iranian supply. The surrounding region near Nishapur is riddled with mines. Turquoise from there, the sky blue or robin egg blue of Persian stone, became the benchmark against which other turquoise quality is judged. Weathered, broken trachyte hosts it. Miners find it in place between layers of limonite and sandstone and also among the loose scree at the foot of the mountain. This deposit is naturally blue and turns green when heated, because the stone dehydrates. Iran also has mines in Semnan and Kerman provinces.

  Iran has supplied turquoise for at least 2,000 years, and its workings are the oldest known, along with those in Sinai. In Iranian architecture the intense blue covered the domes of palaces, where it stood for heaven on earth.

  In Sinai, the First Dynasty of ancient Egypt, around 3000 BCE, is the earliest firm date for mining, and miners may have worked there even earlier. The native Monitu called the region the Country of Turquoise. Six mines lie along the southwest coast of the peninsula, spread over about 650 km2. Two matter most historically, Serabit el-Khadim and Wadi Maghareh, and both are believed to be among the oldest mines anywhere. Serabit el-Khadim lies about 4 kilometres from an ancient temple dedicated to Hathor.

  Large-scale mining no longer pays there. Bedouin miners quarry the deposits sporadically, using homemade gunpowder. Flash floods threaten them in the rainy winter months, and even in the dry season the haphazardly dug sandstone walls can collapse and kill. Sinai stone, often called Egyptian turquoise, is typically greener than the Iranian kind, though it is thought to be stable and fairly durable. It is usually the most translucent of all. Under magnification its surface is peppered with dark blue discs that appear in no other locality.

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• 5. The American Southwest

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  Cerrillos, New Mexico, is thought to hold the oldest mines in the United States. Pre-Columbian Native Americans worked the deposits of New Mexico and California with stone tools, some made locally and some brought from as far away as central Mexico. Before the 1920s New Mexico was the nation's largest producer. Its deposits are now more or less exhausted. Only one California mine, at Apache Canyon, operates commercially today.

  Most American turquoise comes as thin seams and small nuggets, and much of it is poor. Called chalk turquoise, it has high iron levels, so greens and yellows dominate. In its untreated state it is crumbly and unfit for jewelry. Some fine material does turn up that rivals Iranian stone in color and durability.

  Arizona is currently the top producer by value. Two of its mines, famed for their color and quality, were considered the best in the industry. The Sleeping Beauty Mine in Globe stopped mining turquoise in August 2012. Its owners sent all the ore to the crusher to focus on copper, because the world price of copper was rising. The price of natural, untreated Sleeping Beauty turquoise has climbed sharply since. The Kingman Mine was still running beside a copper mine outside the city as of 2015. Other Arizona mines, including Blue Bird, Castle Dome, and Ithaca Peak, are mostly idle because of high operating costs and federal regulations.

  Nevada is the other major producer, with more than 120 mines that have yielded significant turquoise. Unlike most American mines, Nevada's were worked mainly for gem turquoise rather than as a byproduct of other mining. The stone comes as nuggets, as fracture fillings, and as cement between rock fragments in breccias. Because of the local geology, most of it is hard and dense enough to need no treatment. Lander and Esmeralda counties are the chief producers, and most deposits lie along a belt of thrust faulting that stretches from northern Elko County south to the California border.

  Nevada turquoise comes in blues, blue-greens, and greens. Zinc and iron may cause the brightest green to yellow-green shades, though some of those stones may actually be variscite or faustite. Many pieces show brown or black limonite veining, which the trade calls spiderweb matrix. Since the 1870s Nevada has produced an estimated 600 short tons of turquoise, including nearly 400 short tons from the Carico Lake mine. Small operations continue at the Godber, Orvil Jack, and Carico Lake mines in Lander County, and at Pilot Mountain in Mineral County.

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• 6. Beads, Booms and Busts

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  Beads from the Eastern Rhodopes in Bulgaria show people using turquoise as early as the fifth millennium BCE. The raw material may be linked to the nearby Spahievo lead-zinc ore field. In Spain, turquoise appears as a minor mineral in the variscite deposits that prehistoric people exploited at Palazuelos de las Cuevas in Zamora and Can Tintorer in Gavá, near Barcelona. In Greece, beads and small reclining calves made of turquoise date to the Mycenaean era, around 1500 BC.

  China has been a minor source for 3,000 years or more. Gem-quality nodules turn up in fractured, silicified limestone at Yunxian and Zhushan in Hubei province, and Marco Polo reported turquoise in what is now Sichuan. Most Chinese stone is exported, though a few carvings exist that are worked much like jade. In Tibet, gem-quality deposits reportedly lie in the mountains of Derge in the east and Nagari-Khorsum in the west. Other localities include Afghanistan, Victoria and Queensland in Australia, northern India, Chuquicamata in northern Chile, Cornwall, Saxony, Silesia, and Turkestan.

  Turquoise was among the first gems ever mined. Many historic sites are depleted, and the ones still worked are small, remote, and often seasonal. Most are dug by hand with little or no machinery. The exception is the United States, where turquoise is often recovered as a byproduct of big copper operations.

  Mining has also had its extravagant moments. During the reign of Mohammad Khodabanda, from 1578 to 1587, dust that had piled up over fifty years of mining in Safavid Iran was squandered lavishly. That royal excess came amid economic hardship, political discord, and rising factionalism among the qezelbash elite. Centuries later, European interest in turquoise mining flared briefly in the late 1800s. Prices peaked in 1890, then collapsed by 1912, and large-scale operations ended.

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• 7. Wearing the Sky

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  Tutankhamun's tomb gave the modern world its best-known turquoise, above all the pharaoh's iconic burial mask, which was liberally inlaid with the stone. Egyptians had used turquoise since the First Dynasty, and perhaps earlier. They set it in gold as beads and inlay, and wore it in rings and great sweeping pectoral necklaces. Often it was carved as a scarab, alongside carnelian, lapis lazuli, and, in later pieces, colored glass. The goddess Hathor, patroness of Serabit el-Khadim, carried the titles Lady of Turquoise, Mistress of Turquoise, and Lady of Turquoise Country.

  In Persia, turquoise was the de facto national stone for millennia. Craftsmen used it on objects from turbans to bridles, and on mosques and other major buildings, inside and out. One example is the Medresseh-i Shah Husein Mosque of Isfahan. Persian stone was often engraved with devotional words in Arabic script, then inlaid with gold. The style traveled to India with the Mughal Empire, and shows in high-purity gold jewellery set with ruby and diamond, and in the Taj Mahal. India apparently did not know the stone before the Mughals, and Japan did not know it until the 18th century.

  Across the ancient world, many cultures believed turquoise could protect its wearer. It was thought to change color with the wearer's health and to ward off harmful forces. In Europe, turquoise probably arrived through Turkey with other Silk Road novelties. It still did not become important as an ornamental stone in the West until the 14th century, after the Roman Catholic Church's influence declined and allowed its use in secular jewellery.

  The Aztecs saw turquoise as an embodiment of fire, with heat and smokiness as its properties. They inlaid it with gold, quartz, malachite, jet, jade, coral, and shells into mosaic objects that were presumably ceremonial: masks, knives, and shields. Some masks had a human skull as their base. Natural resins, bitumen, and wax bonded the stone to a base that was usually wood, though bone and shell also served.

  The Pueblo, Navajo, and Apache peoples prized turquoise for protection. The Apache believe it gives an archer dead aim, and in Navajo culture it brings, in the words of the source tradition, a spiritual protection and blessing. They made mosaic inlay, sculpture, toroidal beads, and freeform pendants. The Ancestral Puebloans of Chaco Canyon are believed to have prospered greatly from producing and trading turquoise objects. The silver jewellery now associated with the Navajo and other Southwestern tribes is a fairly modern development, thought to date from around 1880.

  In Tibet and Mongolia, cabochons of imported turquoise and coral still decorate silver and gold jewellery, and a greener hue is said to be preferred. Most pieces made there today are inexpensive exports for Western buyers. In the West, French excavations in Egypt from the mid-19th century into the early 20th, including that of Tutankhamun's tomb, stirred public fascination. The stone, already fashionable for its pastel shades since about 1810, became a staple of Egyptian Revival pieces. Today it is most often a cabochon in a silver ring or bracelet, or a tumbled bead in a chunky necklace. The Zuni carve lesser material into fetishes.

  Turquoise is the traditional Western birthstone for December. It is one of the stones in the Jewish High Priest's breastplate, described in Exodus chapter 28. The Zuni and Pueblo regard it as sacred, and the Aztec and Maya also counted it as valuable and culturally important.

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• 8. Fakes, Fixes and Care

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  The Egyptians made the first artificial imitation of turquoise, a glazed earthenware called faience, which makes turquoise arguably the first gemstone ever copied. Later makers used glass and enamel, and in modern times porcelain, plastics, and pressed, bonded, and sintered products. Viennese turquoise is precipitated aluminium phosphate colored with copper oleate. Neolith mixes bayerite with copper phosphate. In 1972 Pierre Gilson introduced a product close to a true synthetic. A binder changes its chemistry, so it is best called a simulant. Gilson turquoise comes in a uniform color or with black veining like natural Nevada stone.

  Today the most common imitation is dyed howlite or magnesite, both white naturally. Howlite also has convincing natural black veining. Dyed chalcedony, jasper, and marble are less common and less convincing. Variscite, faustite, chrysocolla, lazulite, smithsonite, hemimorphite, and wardite are sometimes mistaken for turquoise. So is odontolite, a fossil bone or tooth tinted blue by vivianite, which was once mined in large quantities in southern France to stand in for the real stone.

  Gemologists spot fakes mostly by looking closely at the surface under magnification. Natural turquoise shows a featureless pale blue background peppered with whitish flecks. Manufactured stone is usually a uniform dark blue and granular or sugary in texture. Glass and plastic are far more translucent, often with bubbles or flow lines. Some tests destroy a little material. Diluted hydrochloric acid makes odontolite and magnesite fizz and turns howlite green, and a heated probe can release the pungent smell of plastic.

  Many genuine stones are treated. Waxing and oiling are the oldest methods, used since ancient times to deepen color and lustre, and tradition largely accepts them. Waxed stones can sweat under gentle heat or strong sun and may develop a white bloom. Because fine turquoise often occurs in thin seams, it is glued to a stronger base. This backing is standard for all thinly cut turquoise in the Southwestern United States, where wearers found that unbacked slabs often crack. In the 1980s the electrical engineer and dealer James E. Zachery created a proprietary process to stabilize medium to high-grade stone. Energy-dispersive X-ray spectroscopy can sometimes reveal it through elevated potassium, but in some cases it is undetectable.

  Dyeing with Prussian blue and other dyes is regarded as fraud by some purists, because dyes may fade or rub off on the wearer. Stabilization uses pressure to force epoxy, plastics such as polystyrene, or water glass into chalky stone. The epoxy technique was first developed in the 1950s and is credited to Colbaugh Processing of Arizona, which still operates. The most extreme treatment is reconstitution, where fragments are powdered and bonded with resin. Much of the material sold as reconstituted turquoise, called block, is made entirely of resins and dyes.

  Hardness and richness of color drive value. The most desirable shade is a strong sky blue to robin egg blue, a reference to the eggs of the American robin. Veining is prized in the Southwest and Far East but is not valued in the Near East, where unblemished stone is the ideal. Like coral, turquoise is sold by size in millimetres rather than by weight. Untreated stone always commands a higher price, and bonded and reconstituted material is worth considerably less.

  As a phosphate mineral, turquoise is fragile and sensitive to solvents. Perfume, cosmetics, skin oils, and most commercial cleaning fluids can attack the finish. Prolonged direct sunlight may discolor or dehydrate it. Owners are advised to apply sunscreen and hair spray before putting on turquoise jewellery, to avoid wearing it to the beach, to wipe it with a soft cloth, and to store it in its own container. An airtight container can harm it too.

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