Turquoise

Last updated
Turquoise
Turquoise-40031.jpg
General
Category Phosphate minerals
Formula
(repeating unit)
CuAl6(PO4)4(OH)8·4H2O
Strunz classification 8.DD.15
Crystal system Triclinic
Crystal class Pinacoidal (1)
(same H–M symbol)
Identification
Colour Turquoise, blue, blue-green, green
Crystal habit Massive, nodular
Cleavage Good to perfect_usually N/A
Fracture Conchoidal
Mohs scale hardness5–6
Luster Waxy to subvitreous
Streak Bluish white
Specific gravity 2.6–2.9
Optical propertiesBiaxial (+)
Refractive index nα = 1.610
nβ = 1.615
nγ = 1.650
Birefringence +0.040
Pleochroism Weak
Fusibility Fusible in heated HCl
Solubility Soluble in HCl
References [1] [2] [3]

Turquoise is an opaque, blue-to-green mineral that is a hydrated phosphate of copper and aluminium, with the chemical formula Cu Al 6(PO4)4(OH)8·4H2O . It is rare and valuable in finer grades and has been prized as a gemstone and ornamental stone for thousands of years owing to its unique hue. In recent times, turquoise has been devalued, like most other opaque gems, by the introduction onto the market of treatments, imitations and synthetics.

Mineral Element or chemical compound that is normally crystalline and that has been formed as a result of geological processes

A mineral is, broadly speaking, a solid chemical compound that occurs naturally in pure form. A rock may consist of a single mineral, or may be an aggregate of two or more different minerals, spacially segregated into distinct phases. Compounds that occur only in living beings are usually excluded, but some minerals are often biogenic and/or are organic compounds in the sense of chemistry. Moreover, living beings often synthesize inorganic minerals that also occur in rocks.

In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understood.

Phosphate salt or ester of phosphoric acid

A phosphate is a chemical derivative of phosphoric acid. The phosphate ion is an inorganic chemical, the conjugate base that can form many different salts. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Of the various phosphoric acids and phosphates, organic phosphates are important in biochemistry and biogeochemistry, and inorganic phosphates are mined to obtain phosphorus for use in agriculture and industry. At elevated temperatures in the solid state, phosphates can condense to form pyrophosphates.

Contents

The gemstone has been known by many names. Pliny the Elder referred to the mineral as callais (from Ancient Greek κάλαϊς) and the Aztecs knew it as chalchihuitl. [4] The word turquoise dates to the 17th century and is derived from the French turquois meaning "Turkish" because the mineral was first brought to Europe through Turkey, from mines in the historical Khorasan Province of Persia. [2] [3] [4] [5]

Pliny the Elder Roman military commander and writer

Pliny the Elder was a Roman author, a naturalist and natural philosopher, a naval and army commander of the early Roman Empire, and a friend of emperor Vespasian.

Ancient Greek Version of the Greek language used from roughly the 9th century BCE to the 6th century CE

The Ancient Greek language includes the forms of Greek used in Ancient Greece and the ancient world from around the 9th century BCE to the 6th century CE. It is often roughly divided into the Archaic period, Classical period, and Hellenistic period. It is antedated in the second millennium BCE by Mycenaean Greek and succeeded by medieval Greek.

French language Romance language

French is a Romance language of the Indo-European family. It descended from the Vulgar Latin of the Roman Empire, as did all Romance languages. French evolved from Gallo-Romance, the spoken Latin in Gaul, and more specifically in Northern Gaul. Its closest relatives are the other langues d'oïl—languages historically spoken in northern France and in southern Belgium, which French (Francien) has largely supplanted. French was also influenced by native Celtic languages of Northern Roman Gaul like Gallia Belgica and by the (Germanic) Frankish language of the post-Roman Frankish invaders. Today, owing to France's past overseas expansion, there are numerous French-based creole languages, most notably Haitian Creole. A French-speaking person or nation may be referred to as Francophone in both English and French.

Properties

The finest of turquoise reaches a maximum Mohs hardness of just under 6, or slightly more than window glass. [2] Characteristically a cryptocrystalline mineral, turquoise almost never forms single crystals, and all of its properties are highly variable. X-ray diffraction testing shows its crystal system to be triclinic. [3] [6] With lower hardness comes lower specific gravity (2.60–2.90) [3] and greater porosity; these properties are dependent on grain size. The lustre of turquoise is typically waxy to subvitreous, and its 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.

Mohs scale of mineral hardness qualitative ordinal scale characterizing scratch resistance of various minerals

The Mohs scale of mineral hardness is a qualitative ordinal scale characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. Created in 1812 by German geologist and mineralogist Friedrich Mohs, it is one of several definitions of hardness in materials science, some of which are more quantitative. The method of comparing hardness by observing which minerals can scratch others is of great antiquity, having been mentioned by Theophrastus in his treatise On Stones, c. 300 BC, followed by Pliny the Elder in his Naturalis Historia, c. AD 77. While greatly facilitating the identification of minerals in the field, the Mohs scale does not show how well hard materials perform in an industrial setting.

Glass amorphous solid that exhibits a glass transition when heated towards the liquid state

Glass is a non-crystalline, amorphous solid that is often transparent and has widespread practical, technological, and decorative uses in, for example, window panes, tableware, and optoelectronics. The most familiar, and historically the oldest, types of manufactured glass are "silicate glasses" based on the chemical compound silica (silicon dioxide, or quartz), the primary constituent of sand. The term glass, in popular usage, is often used to refer only to this type of material, which is familiar from use as window glass and in glass bottles. Of the many silica-based glasses that exist, ordinary glazing and container glass is formed from a specific type called soda-lime glass, composed of approximately 75% silicon dioxide (SiO2), sodium oxide (Na2O) from sodium carbonate (Na2CO3), calcium oxide (CaO), also called lime, and several minor additives.

Cryptocrystalline is a rock texture made up of such minute crystals that its crystalline nature is only vaguely revealed even microscopically in thin section by transmitted polarized light. Among the sedimentary rocks, chert and flint are cryptocrystalline. Carbonado, a form of diamond, is also cryptocrystalline. Volcanic rocks, especially of the acidic type such as felsites and rhyolites, may have a cryptocrystalline groundmass as distinguished from pure obsidian (acidic) or tachylyte (basic), which are natural rock glasses. Onyx is also a cryptocrystalline.

The refractive index of turquoise (as measured by sodium light, 589.3  nm) 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 nm and a weak band at 460 nm (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.

Refractive index optical characteristic of a material

In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how fast light propagates through the material. It is defined as

Sodium Chemical element with atomic number 11

Sodium is a chemical element with the symbol Na (from Latin natrium) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table, because it has a single electron in its outer shell, which it readily donates, creating a positively charged ion—the Na+ cation. Its only stable isotope is 23Na. The free metal does not occur in nature, and must be prepared from compounds. Sodium is the sixth most abundant element in the Earth's crust and exists in numerous minerals such as feldspars, sodalite, and rock salt (NaCl). Many salts of sodium are highly water-soluble: sodium ions have been leached by the action of water from the Earth's minerals over eons, and thus sodium and chlorine are the most common dissolved elements by weight in the oceans.

The nanometre or nanometer is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre. It can be written in scientific notation as 1×10−9 m, in engineering notation as 1 E−9 m, and as simply 1/1000000000 metres. When used as a prefix for something other than a unit of measure, nano refers to nanotechnology, or phenomena typically occurring on a scale of nanometres.

Turquoise is insoluble in all but heated hydrochloric acid. Its streak is a pale bluish white and its fracture is conchoidal, [3] 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.

Hydrochloric acid strong mineral acid

Hydrochloric acid or muriatic acid is a colorless inorganic chemical system with the formula H
2
O:HCl
. Hydrochloric acid has a distinctive pungent smell. It is classified as strongly acidic and can attack the skin over a wide composition range, since the hydrogen chloride completely dissociates in aqueous solution.

Streak (mineralogy) color of the powder produced when it is dragged across an un-weathered surface

The streak of a mineral is the color of the powder produced when it is dragged across an un-weathered surface. Unlike the apparent color of a mineral, which for most minerals can vary considerably, the trail of finely ground powder generally has a more consistent characteristic color, and is thus an important diagnostic tool in mineral identification. If no streak seems to be made, the mineral's streak is said to be white or colorless. Streak is particularly important as a diagnostic for opaque and colored materials. It is less useful for silicate minerals, most of which have a white streak or are too hard to powder easily.

A fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displacement develops perpendicular to the surface of displacement, it is called a normal tensile crack or simply a crack; if a displacement develops tangentially to the surface of displacement, it is called a shear crack, slip band, or dislocation.

Formation

"Big Blue", a large turquoise specimen from the copper mine at Cananea, Sonora, Mexico Big turquoise from Cananea.jpg
"Big Blue", a large turquoise specimen from the copper mine at Cananea, Sonora, Mexico

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 primary copper sulfides such as chalcopyrite or from the secondary carbonates malachite or azurite; the aluminium may derive from feldspar; and the phosphorus from apatite. Climate factors appear to play an important role as turquoise is typically found in arid regions, filling or encrusting cavities and fractures in typically highly altered volcanic rocks, often with associated limonite and other iron oxides. In the Southwestern United States turquoise is almost invariably associated with the weathering products of copper sulfide deposits in or around potassium-feldspar-bearing porphyritic intrusives. In some occurrences alunite, potassium aluminium sulfate, is a prominent secondary mineral. Typically turquoise mineralization is restricted to a relatively shallow depth of less than 20 metres (66 feet), although it does occur along deeper fracture zones where secondary solutions have greater penetration or the depth to the water table is greater.

Weathering Breaking down of rocks, soil and minerals as well as artificial materials through contact with the Earths atmosphere, biota and waters

Weathering is the breaking down of rocks, soil, and minerals as well as wood and artificial materials through contact with the Earth's atmosphere, water, and biological organisms. Weathering occurs in situ, that is, in the same place, with little or no movement, and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity and then being transported and deposited in other locations.

Copper sulfides describe a family of chemical compounds and minerals with the formula CuSy. Both minerals and synthetic materials comprise these compounds. Some copper sulfides are economically important ores.

Chalcopyrite sulfide mineral

Chalcopyrite ( KAL-ko-PY-ryt) is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical formula CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.

Turquoise is nearly always cryptocrystalline and massive and assumes no definite external shape. Crystals, even at the microscopic scale, are exceedingly rare. Typically the form is vein or fracture filling, nodular, or botryoidal in habit. Stalactite forms have been reported. Turquoise may also pseudomorphously replace feldspar, apatite, other minerals, or even fossils. Odontolite is fossil bone or ivory that has been traditionally thought to have been altered by turquoise or similar phosphate minerals such as the iron phosphate vivianite. Intergrowth with other secondary copper minerals such as chrysocolla is also common.

Occurrence

Massive Kingman blue turquoise in matrix with quartz from the Mineral Park mine, Arizona, US Turquoise with quartz.jpg
Massive Kingman blue turquoise in matrix with quartz from the Mineral Park mine, Arizona, US

Turquoise was among the first gems to be mined, and many historic sites have been depleted, though some are still worked to this day. These are all small-scale operations, often seasonal 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.

Cutting and grinding turquoise in Nishapur, Iran, 1973 Cutting and grinding turquoise in Meshed, Iran. 1973.jpg
Cutting and grinding turquoise in Nishapur, Iran, 1973

Iran

Iran has been an important source of turquoise for at least 2,000 years. It was initially named by Iranians "pērōzah" meaning "victory", and later the Arabs called it "fayrūzah", which is pronounced in Modern Persian as "fīrūzeh". In Iranian architecture, the blue turquoise was used to cover the domes of palaces because its intense blue colour was also a symbol of heaven on earth.[ citation needed ]

Persian turquoise from Iran Persian Turquoise.jpg
Persian turquoise from Iran

This deposit is blue naturally and turns green when heated due to dehydration. It is restricted to a mine-riddled region in Nishapur, the 2,012 m (6,601 ft) mountain peak of Ali-mersai near 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 are the oldest known, together with those of the Sinai Peninsula. [5] Iran also has turquoise mines in Semnan and Kerman provinces.

Sinai

Since at least the First Dynasty (3000  BCE) in ancient Egypt, and possibly before then, turquoise was used by the Egyptians and was mined by them in the Sinai Peninsula. This region was known as the Country of Turquoise by the native Monitu. There are six mines in the peninsula, all on its southwest coast, covering an area of some 650 km2 (250 sq mi). The two most important of these mines, from a historic perspective, are Serabit el-Khadim 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 the deity Hathor.

The turquoise is found in sandstone that is, or was originally, overlain by basalt. Copper and iron workings are present in the area. Large-scale turquoise mining is not profitable today, but the deposits are sporadically quarried by Bedouin peoples using homemade gunpowder.[ citation needed ] In the rainy winter months, miners face a risk from flash flooding; even in the dry season, death from the collapse of the haphazardly 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.

A selection of Ancestral Pueblo (Anasazi) turquoise and orange argillite inlay pieces from Chaco Canyon, New Mexico, US (dated c. 1020-1140) show the typical colour range and mottling of American turquoise. Some likely came from Los Cerrillos. Chacoan turquoise with argillite.jpg
A selection of Ancestral Pueblo (Anasazi) turquoise and orange argillite inlay pieces from Chaco Canyon, New Mexico, US (dated c. 1020–1140) show the typical colour range and mottling of American turquoise. Some likely came from Los Cerrillos.

United States

A fine turquoise specimen from Los Cerrillos, New Mexico, US, at the Smithsonian Museum. Cerrillos turquoise was widely used by Native Americans prior to the Spanish conquest. Turquoise Cerillos Smithsonian.jpg
A fine turquoise specimen from Los Cerrillos, New Mexico, US, at the Smithsonian Museum. Cerrillos turquoise was widely used by Native Americans prior to the Spanish conquest.
Bisbee turquoise commonly has a hard chocolate brown coloured matrix. Turquoisecollection.jpg
Bisbee turquoise commonly has a hard chocolate brown coloured matrix.
Untreated turquoise, Nevada, US. Rough nuggets from the McGinness Mine, Austin. Blue and green cabochons showing spiderweb, Bunker Hill Mine, Royston Turq mcGuin bunker.jpg
Untreated turquoise, Nevada, US. Rough nuggets from the McGinness Mine, Austin. Blue and green cabochons showing spiderweb, Bunker Hill Mine, Royston

The Southwest United States is a significant source of turquoise; Arizona, California (San Bernardino, Imperial, Inyo counties), Colorado (Conejos, El Paso, Lake, Saguache counties), New Mexico (Eddy, Grant, Otero, Santa Fe counties) and Nevada (Clark, Elko, Esmeralda County, Eureka, Lander, Mineral County and Nye counties) 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 is sometimes found, rivalling Iranian material in both colour and durability, most American turquoise is of a low grade (called "chalk turquoise"); high iron levels mean greens and yellows predominate, and a typically friable consistency in the turquoise's untreated state precludes use in jewellery.

Arizona is currently the most important producer of turquoise by value. [5] Several mines exist in the state, two of them famous for their unique colour and quality and considered the best in the industry: the Sleeping Beauty Mine in Globe ceased turquoise mining in August 2012. The mine chose to send all ore to the crusher and to concentrate on copper production due to the rising price of copper on the world market. The price of natural untreated Sleeping Beauty turquoise has risen dramatically since the mine's closing. The Kingman Mine as of 2015 still operates alongside a copper mine outside of the city. Other mines include the Blue Bird mine, Castle Dome, and Ithaca Peak, but they are mostly inactive due to the high cost of operations and federal regulations. The Phelps Dodge Lavender Pit mine at Bisbee ceased operations in 1974 and never had a turquoise contractor. All Bisbee turquoise was "lunch pail" mined. It came out of the copper ore mine in miners' lunch pails. Morenci and Turquoise Peak are either inactive or depleted.

Nevada is the country's other major producer, with more than 120 mines which have yielded significant quantities of turquoise. Unlike elsewhere in the US, most Nevada mines have been worked primarily for their gem turquoise and very little has been recovered as a byproduct of other mining operations. Nevada turquoise is found as nuggets, fracture fillings and in breccias as the cement filling interstices between fragments. Because of the geology of the Nevada deposits, a majority of the material produced is hard and dense, being of sufficient quality that no treatment or enhancement is required. While nearly every county in the state has yielded some turquoise, the chief producers are in Lander and Esmeralda counties. Most of the turquoise deposits in Nevada occur along a wide belt of tectonic activity that coincides with the state's zone of thrust faulting. It strikes at a bearing of about 15° and extends from the northern part of Elko County, southward down to the California border southwest of Tonopah. Nevada has produced a wide diversity of colours and mixes of different matrix patterns, with turquoise from Nevada coming in various shades of blue, blue-green, and green. Some of this unusually-coloured turquoise may contain significant zinc and iron, which is the cause of the beautiful bright green to yellow-green shades. Some of the green to green-yellow shades may actually be variscite or faustite, which are secondary phosphate minerals similar in appearance to turquoise. A significant portion of the Nevada material is also noted for its often attractive brown or black limonite veining, producing what is called "spiderweb matrix". While a number of the Nevada deposits were first worked by Native Americans,[ which? ] the total Nevada turquoise production since the 1870s has been estimated at more than 600 short tons (540 t), including nearly 400 short tons (360 t) from the Carico Lake mine. In spite of increased costs, small scale mining operations continue at a number of turquoise properties in Nevada, including the Godber, Orvil Jack and Carico Lake mines in Lander County, the Pilot Mountain Mine in Mineral County, and several properties in the Royston and Candelaria areas of Esmerelda County. [7]

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 in) 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.[ citation needed ]

In an attempt to recoup profits and meet demand, some 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). There are however, some American mines which produce materials of high enough quality that no treatment or alterations are required. Any such treatments which have been performed should be disclosed to the buyer on sale of the material.

Other sources

Turquoise prehistoric artefacts (beads) are known since the fifth millennium BCE from sites in the Eastern Rhodopes in Bulgaria – the source for the raw material is possibly related to the nearby Spahievo lead–zinc ore field. [8]

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, gem-quality deposits purportedly exist in the mountains of Derge and Nagari-Khorsum in the east and west of the region respectively. [9]

Other notable localities include: Afghanistan; Australia (Victoria and Queensland); north India; northern Chile (Chuquicamata); Cornwall; Saxony; Silesia; and Turkestan.

History of use

Trade in turquoise crafts, such as this freeform pendant dating from 1000-1040, is believed to have brought the Ancestral Pueblo people of the Chaco Canyon great wealth. Chacoan turquoise pendant.jpg
Trade in turquoise crafts, such as this freeform pendant dating from 1000–1040, is believed to have brought the Ancestral Pueblo people of the Chaco Canyon great wealth.
Moche turquoise nose ornament. Larco Museum Collection, Lima, Peru Turqoise nose ornament.jpg
Moche turquoise nose ornament. Larco Museum Collection, Lima, Peru
Backswords, inlaid with turquoise. Russia, 17th century. Drevnosti RG v3 ill094 - Palash.jpg
Backswords, inlaid with turquoise. Russia, 17th century.
Turquoise mosaic mask of Xiuhtecuhtli, the Aztec god of fire. Xiuhtecuhtli (mask).jpg
Turquoise mosaic mask of Xiuhtecuhtli, the Aztec god of fire.
The iconic gold burial mask of Tutankhamun, inlaid with turquoise, lapis lazuli, carnelian and coloured glass. Tutmask.jpg
The iconic gold burial mask of Tutankhamun, inlaid with turquoise, lapis lazuli, carnelian and coloured glass.

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. [10] 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 Roman Catholic Church's influence which allowed the use of turquoise in secular jewellery. It was apparently unknown in India until the Mughal period, and unknown in Japan until the 18th century. A common belief shared by many of these civilizations 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 shells, 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 around 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,[ citation needed ] 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 purity 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.

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 Ancient 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 structure created by an artificial glazed ceramic product known as faience.

The French conducted archaeological excavations of Egypt from the mid-19th century through the early 20th. These excavations, including that of Tutankhamun's tomb, created great public interest in the western world, subsequently influencing jewellery, architecture, and art of the time. Turquoise, already favoured for its pastel shades since around 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.

Cultural associations

In many cultures of the Old and New Worlds, this gemstone has been esteemed for thousands of years as a holy stone, a bringer of good fortune or a talisman. The oldest evidence for this claim was found in Ancient Egypt, where grave furnishings with turquoise inlay were discovered, dating from approximately 3000 BCE. In the ancient Persian Empire, the sky-blue gemstones were earlier worn round the neck or wrist as protection against unnatural death. If they changed colour, the wearer was thought to have reason to fear the approach of doom. Meanwhile, it has been discovered that the turquoise certainly can change colour, but that this is not necessarily a sign of impending danger. The change can be caused by the light, or by a chemical reaction brought about by cosmetics, dust or the acidity of the skin. [11]

The goddess Hathor was associated with turquoise, as she was the patroness of Serabit el-Khadim, where it was mined. Her titles included "Lady of Turquoise", "Mistress of Turquoise", and "Lady of Turquoise Country". [12]

In Western culture, turquoise is also the traditional birthstone for those born in the month of December. The turquoise is also a stone in the Jewish High Priest's breastplate, described in Exodus chapter 28. The stone is also considered sacred to the indigenous Zuni and Pueblo peoples of the American Southwest. [13] [14] The pre-Columbian Aztec and Maya also considered it to be a valuable and culturally important stone. [15]

Imitations

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

The Egyptians were the first to produce an artificial imitation of turquoise, in the glazed earthenware product faience. Later glass and enamel were also used, and in modern times more sophisticated 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(II) 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.

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 and faustite; [5] 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 blue 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 pungent 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.

Treatments

An early turquoise mine in Madan, Khorasan Province, Iran Madan Turquoise Mines.jpg
An early turquoise mine in Madan, Khorasan Province, Iran

Turquoise is treated to enhance both its colour and durability (increased hardness and decreased porosity). As is so often the case with any precious stones, full disclosure about treatment is frequently not given. Gemologists can detect these treatments using a variety of testing methods, some of which are destructive, such as the use of a heated probe applied to an inconspicuous spot, which will reveal oil, wax or plastic treatment.

Waxing and oiling

Historically, light waxing and oiling were the first treatments used in ancient times, providing a wetting effect, thereby enhancing the colour and lustre. This treatment is more or less acceptable by tradition, especially because treated turquoise is usually of a higher grade to begin with. Oiled and waxed stones are 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.)

Stabilization

Material treated with plastic or water glass is termed "bonded" or "stabilized" turquoise. This process consists of pressure impregnation of otherwise unsaleable chalky American material by epoxy and plastics (such as polystyrene) and water glass (sodium silicate) to produce a wetting effect and improve durability. Plastic and water glass treatments are far more permanent and stable than waxing and oiling, and can be applied to material too chemically or physically unstable for oil or wax to provide sufficient improvement. Conversely, stabilization and bonding are rejected by some as too radical an alteration. [16]

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.[ citation needed ]

Dyeing

The use of Prussian blue and other dyes (often in conjunction with bonding treatments) to "enhance” its appearance, make uniform or completely change the colour, is regarded as fraudulent by some purists, [16] especially since some dyes may fade or rub off on the wearer. Dyes have also been used to darken the veins of turquoise.

Reconstitution

Perhaps the most extreme of treatments is "reconstitution", wherein fragments of fine turquoise material, too small to be used individually, are powdered and then bonded with resin to form a solid mass. Very often the material sold as "reconstituted turquoise" is artificial, with little or no natural stone, made entirely from resins and dyes. In the trade reconstituted turquoise is often called "block turquoise" or simply "block".

Backing

Since finer turquoise is often found as thin seams, it may be glued to a base of stronger foreign material for reinforcement. These stones are termed "backed," and it is standard practice that all thinly cut turquoise in the Southwestern United States is backed. Native indigenous peoples of this region, because of their considerable use and wearing of turquoise, have found that backing increases the durability of thinly cut slabs and cabochons of turquoise. They observe that if the stone is not backed it will often crack. Early backing materials included the casings of old model T batteries, old phonograph records, and more recently epoxy steel resins. Backing of turquoise is not widely known outside of the Native American and Southwestern United States jewellery trade. Backing does not diminish the value of high quality turquoise, and indeed the process is expected for most thinly cut American commercial gemstones.[ citation needed ]

Valuation and care

Slab of turquoise in matrix showing a large variety of different colouration Turquoise-slab.jpg
Slab of turquoise in matrix showing a large variety of different colouration

Hardness and richness of colour are two of the major factors in determining the value of turquoise; while colour is a matter of individual taste, generally speaking, the most desirable is a strong sky to robin egg blue (in reference to the eggs of the American robin). [9] Whatever the colour, for many applications, turquoise should not be 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 complementary, 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 complementary 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 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, as will most commercial jewellery cleaning fluids. Prolonged exposure to direct sunlight may also discolour or dehydrate turquoise. Care should therefore be taken when wearing such jewels: cosmetics, including sunscreen and hair spray, 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 buildup of residue, and should be stored in its own container to avoid scratching by harder gems. Turquoise can also be adversely affected if stored in an airtight container.

See also

Related Research Articles

Emerald green gemstone, a beryl variety

Emerald is a gemstone and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium and sometimes vanadium. Beryl has a hardness of 7.5–8 on the Mohs scale. Most emeralds are highly included, so their toughness (resistance to breakage) is classified as generally poor. Emerald is a cyclosilicate.

Gemstone Piece of mineral crystal used to make jewelry

A gemstone is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks and occasionally organic materials that are not minerals are also used for jewelry and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone.

Jewellery Form of personal adornment

Jewellery or jewelry consists of small decorative items worn for personal adornment, such as brooches, rings, necklaces, earrings, pendants, bracelets, and cufflinks. Jewellery may be attached to the body or the clothes. From a western perspective, the term is restricted to durable ornaments, excluding flowers for example. For many centuries metal, often combined with gemstones, has been the normal material for jewellery, but other materials such as shells and other plant materials may be used. It is one of the oldest type of archaeological artefact – with 100,000-year-old beads made from Nassarius shells thought to be the oldest known jewellery. The basic forms of jewellery vary between cultures but are often extremely long-lived; in European cultures the most common forms of jewellery listed above have persisted since ancient times, while other forms such as adornments for the nose or ankle, important in other cultures, are much less common.

Sapphire gemstone

Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide (α-Al2O3) with trace amounts of elements such as iron, titanium, chromium, copper, or magnesium. It is typically blue, but natural "fancy" sapphires also occur in yellow, purple, orange, and green colors; "parti sapphires" show two or more colors. The only color corundum stone that the term sapphire is not used for is red, which is called a ruby. Pink colored corundum may be either classified as ruby or sapphire depending on locale. Commonly, natural sapphires are cut and polished into gemstones and worn in jewelry. They also may be created synthetically in laboratories for industrial or decorative purposes in large crystal boules. Because of the remarkable hardness of sapphires – 9 on the Mohs scale (the third hardest mineral, after diamond at 10 and moissanite at 9.5) – sapphires are also used in some non-ornamental applications, such as infrared optical components, high-durability windows, wristwatch crystals and movement bearings, and very thin electronic wafers, which are used as the insulating substrates of very special-purpose solid-state electronics (especially integrated circuits and GaN-based LEDs).

Topaz Silicate mineral

Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al2SiO4(F, OH)2. Topaz crystallizes in the orthorhombic system, and its crystals are mostly prismatic terminated by pyramidal and other faces. It is one of the hardest naturally occurring minerals (Mohs hardness of 8) and is the hardest of any silicate mineral. This hardness combined with its usual transparency and variety of colors means that it has acquired wide use in jewellery as a cut gemstone as well as for intaglios and other gemstone carvings.

Tourmaline Cyclosilicate mineral group

Tourmaline is a crystalline boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gemstone comes in a wide variety of colors.

Garnet mineral, semi-precious stone

Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.

Ruby variety of corundum, mineral, gemstone

A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum. Other varieties of gem-quality corundum are called sapphires. Ruby is one of the traditional cardinal gems, together with amethyst, sapphire, emerald, and diamond. The word ruby comes from ruber, Latin for red. The color of a ruby is due to the element chromium.

Lapis lazuli A contact metamorphic rock containing lazurite, pyrite and calcite

Lapis lazuli, or lapis for short, is a deep blue metamorphic rock used as a semi-precious stone that has been prized since antiquity for its intense color. As early as the 7th millennium BCE, lapis lazuli was mined in the Sar-i Sang mines, in Shortugai, and in other mines in Badakhshan province in northeast Afghanistan. Lapis was highly valued by the Indus Valley Civilisation. Lapis beads have been found at Neolithic burials in Mehrgarh, the Caucasus, and even as far from Afghanistan as Mauritania. It was used in the funeral mask of Tutankhamun.

Tigers eye A chatoyant gemstone that is usually a metamorphic rock with a golden to red-brown colour and a silky lustre

Tiger's eye is a chatoyant gemstone that is usually a metamorphic rock with a golden to red-brown colour and a silky lustre. As members of the quartz group, tiger's eye and the related blue-coloured mineral hawk's eye gain their silky, lustrous appearance from the parallel intergrowth of quartz crystals and altered amphibole fibres that have mostly turned into limonite.

Variscite phosphate mineral

Variscite is a hydrated aluminium phosphate mineral (AlPO4·2H2O). It is a relatively rare phosphate mineral. It is sometimes confused with turquoise; however, variscite is usually greener in color. The green color results from the presence of small amounts of chromium.

Chrysocolla phyllosilicate mineral

Chrysocolla is a hydrated copper phyllosilicate mineral with formula: Cu2−xAlx(H2−xSi2O5)(OH)4·nH2O (x<1) or (Cu,Al)2H2Si2O5(OH)4·nH2O. The structure of the mineral has been questioned, as spectrographic studies suggest material identified as chrysocolla may be a mixture of the copper hydroxide spertiniite and chalcedony.

Brazilianite phosphate mineral

Brazilianite, whose name derives from its country of origin, Brazil, is a typically yellow-green phosphate mineral, most commonly found in phosphate-rich pegmatites.

Priestly breastplate

The priestly breastplate was a sacred breastplate worn by the High Priest of the Israelites, according to the Book of Exodus. In the biblical account, the breastplate is sometimes termed the breastplate of judgment, because the Urim and Thummim were placed within it. These stones were, at times, used to determine God’s will in a particular situation. Using these stones did not always determine God's will

Turquoise is a blue/green color, based on the gem of the same name. The word turquoise comes from the French for "Turkish", as the gem was originally imported from Turkey. The first recorded use of turquoise as a color name in English was in 1573.

Prasiolite Mineral, quartz variety

Prasiolite is a green variety of quartz, a silicate mineral chemically silicon dioxide.

Howlite inoborate mineral

Howlite, a calcium borosilicate hydroxide (Ca2B5SiO9(OH)5), is a borate mineral found in evaporite deposits.

Ammolite

Ammolite is an opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral contained in nacre, with a microstructure inherited from the shell. It is one of few biogenic gemstones; others include amber and pearl.1 In 1981, ammolite was given official gemstone status by the World Jewellery Confederation (CIBJO), the same year commercial mining of ammolite began. It was designated the official gemstone of the City of Lethbridge, Alberta in 2007.

Hardstone carving

Hardstone carving is a general term in art history and archaeology for the artistic carving of predominantly semi-precious stones, such as jade, rock crystal, agate, onyx, jasper, serpentine, or carnelian, and for an object made in this way. Normally the objects are small, and the category overlaps with both jewellery and sculpture. Hardstone carving is sometimes referred to by the Italian term pietre dure; however, pietra dura is the common term used for stone inlay work, which causes some confusion.

Mining is an important industry in Pakistan. Pakistan has deposits of several minerals including coal, copper, gold, chromite, mineral salt, bauxite and several other minerals. There are also a variety of precious and semi-precious minerals that are also mined. These include peridot, aquamarine, topaz, ruby, emerald, rare-earth minerals bastnaesite and xenotime, sphene, tourmaline, and many varieties and types of quartz .

References

  1. Hurlbut, Cornelius S.; Klein, Cornelis (1985). Manual of Mineralogy (20th ed.). New York, NY: John Wiley and Sons. ISBN   978-0-471-80580-9.
  2. 1 2 3 "Turquoise: mineral information and data". mindat.org. Archived from the original on 2006-11-12. Retrieved 2006-10-04.
  3. 1 2 3 4 5 Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (2000). "Turquoise" (PDF). Handbook of Mineralogy. IV. Chantilly, VA: Mineralogical Society of America. ISBN   978-0-9622097-3-4. Archived (PDF) from the original on 2012-02-11.
  4. 1 2 Palache, C.; Berman, H.; Frondel, C. (1951). Dana's System of Mineralogy. II (7th ed.). Wiley. pp. 946–951.
  5. 1 2 3 4 Turquoise Archived 2007-05-20 at the Wayback Machine . minerals.usgs.gov
  6. "Turquoise R050554". RRUFF™ Project. Archived from the original on 2017-05-02.
  7. Minerals of Nevada. Special Publication 31. Nevada Bureau of Mines. pp. 78–81, 443–445.
  8. Kostov, R. I.; Chapman, J.; Gaydarska, B.; Petrov, I.; Raduntcheva, A. (2007). "Turquoise – archaeomineralogical evidences from the Orlovo prehistoric site (Haskovo district, Southern Bulgaria)". Geology and Mineral Resources. 14 (7–8): 17–22.
  9. 1 2 Watson, Martin. "Turquoise – The Gemstone of Tibet". Archived from the original on March 19, 2007. Retrieved 2007-06-01.
  10. "China Exhibition". Washington, DC: National Gallery of Art. 1999. Archived from the original on 2006-09-30. Retrieved 2006-09-23.
  11. Shepardson, Mathew. "Does Turquoise Change Color?". T. Skies. Archived from the original on 11 March 2018. Retrieved 11 March 2018.
  12. The Supreme Council of Antiquities (2007). Bulletin of the Egyptian Museum. p. 24.
  13. Lowry, Joe Dan. Turquoise. p. 36–41.[ full citation needed ]
  14. Sharing the Art of New Mexico. Collector's Guide. 17. 2003. p. 184.
  15. Lowry, Joe Dan. Turquoise. p. 34.[ full citation needed ]
  16. 1 2 Harriss, Joseph A. "Tantalizing Turquoise". Archived from the original on 2008-02-01. Retrieved 2007-06-01.

Further reading