Therobin egg blue orsky blue color of thePersian turquoise mined near the modern city ofNishapur,Iran, has been used as a guiding reference for evaluating turquoise quality.[5]
Like most other opaque gems, turquoise has been devalued by the introduction of treatments, imitations, and synthetics into the market.
The wordturquoise dates to the 17th century and is derived from theOld Frenchturquois meaning "Turkish" because the mineral was first brought to Europe through theOttoman Empire from the mines in the historicalKhorasan province ofIran (Persia).[6][3][4][7][8] However, according toEtymonline, the word dates to the 14th century with the formturkeis, meaning "Turkish", which was replaced withturqueise from French in the 1560s. According to the same source, the gemstone was first brought to Europe fromTurkestan or another Turkic territory.[9]Pliny the Elder referred to the mineral ascallais (fromAncient Greekκάλαϊς) and theAztecs knew it aschalchihuitl.[7]
In professional mineralogy, until the mid-19th century, the scientific nameskalaite[10]: 571–572 orazure spar were also used, which simultaneously provided a version of the mineral origin of turquoise.[11]: 452 However, these terms did not become widespread and gradually fell out of use.
Turquoise mining in New Mexico'sCerrillos Hills began with Native Americans, later attracting brief European interest in the late 1800s. Prices peaked in 1890, then collapsed by 1912, ending large-scale operations.[12] DuringMohammad Khodabanda reign (1578–1587), accumulated turquoise dust from fifty years of mining inSafavid Iran was squandered lavishly, reflecting royal excess amid economic hardship, political discord, and rising factionalism among the qezelbash elite.[13]
The finest of turquoise reaches a maximumMohs hardness of just under 6, or slightly more than windowglass.[3] Characteristically acryptocrystalline mineral, turquoise almost never forms singlecrystals, and all of its properties are highly variable.X-ray diffraction testing shows itscrystal system to betriclinic.[4][14] With lower hardness comes greaterporosity.[15] Thelustre of turquoise is typically waxy to subvitreous, and itstransparency 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 toidiochromatic copper[16] while the green may be the result ofiron impurities (replacing copper.)[17]: 29
Therefractive index of turquoise varies from 1.61 to 1.65 on the three crystal axes, withbirefringence 0.040, biaxial positive, as measured from rare single crystals.[2]
Crushed turquoise is soluble in hothydrochloric acid.[15] Itsstreak is white to greenish to blue, and itsfracture is smooth toconchoidal.[4] Despite its low hardness relative to other gems, turquoise takes a good polish. Turquoise may also be peppered with flecks ofpyrite or interspersed with dark, spiderylimonite veining.
Turquoise is nearly always cryptocrystalline and massive and assumes no definite external shape. Crystals, even at the microscopic scale, are rare. Typically the form is a vein or fracture filling, nodular, orbotryoidal inhabit.[2]Stalactite forms have been reported. Turquoise may alsopseudomorphously replace feldspar, apatite, other minerals, or evenfossils.Odontolite is fossil bone orivory that has historically been thought to have been altered by turquoise or similar phosphate minerals such as the iron phosphatevivianite. Intergrowth with other secondary copper minerals such aschrysocolla is also common. Turquoise is distinguished from chrysocolla, the only common mineral with similar properties, by its greater hardness.[2]
Turquoise forms a completesolid solution series withchalcosiderite,CuFe6(PO4)4(OH)8·4H2O, in whichferric iron replaces aluminium.[2]
Turquoise deposits probably form in more than one way.[18] However, a typical turquoise deposit begins withhydrothermal deposition ofcopper sulfides. This takes place when hydrothermal fluids leach copper from a host rock, which is typically anintrusion ofcalc-alkaline rock with a moderate to highsilica content that is relativelyoxidized. The copper is redeposited in more concentrated form as acopper porphyry, in which veins of copper sulfide fill joints and fractures in the rock. Deposition takes place mostly in the potassic alteration zone, which is characterized by conversion of existingfeldspar topotassium feldspar and deposition ofquartz andmicas at a temperature of 400–600 °C (752–1,112 °F).[4][19][20]
Turquoise is asecondary orsupergene mineral, not present in the original copper porphyry.[4] It forms whenmeteoric water (rain or snow melt infiltrating theEarth's surface) percolates through the copper porphyry. Dissolved oxygen in the water oxidizes the copper sulfides to soluble sulfates, and the acidic, copper-laden solution then reacts with aluminum and potassium minerals in the host rock to precipitate turquoise.[21] This typically fills veins in volcanic rock or phosphate-rich sediments.[4] Deposition usually takes place at a relatively low temperature, 90–195 °C (194–383 °F), and seems to occur more readily in arid environments.[21]
Turquoise in theSinai Peninsula is found inlower Carboniferoussandstones overlain bybasalt flows andupper Carboniferouslimestone. The overlying beds were presumably the source of the copper, which precipitated as turquoise in nodules, horizontal seams, or vertical joints in the sandstone beds. The classical Iranian deposits are found in sandstones and limestones ofTertiary age that were intruded byapatite-richporphyritictrachytes andmafic rock. Supergene alteration fractured the rock and converted some of the minerals in the rock toalunite, which freed aluminum and phosphate to combine with copper from oxidized copper sulfides to form turquoise. This process took place at a relatively shallow depth, and by 1965 the mines had "bottomed" at a depth averaging just 9 meters (30 ft) below the surface.[21]
Turquoise deposits are widespread in North America. Some deposits, such as those ofSaguache andConejos Counties in Colorado[21] or theCerrillos Hills in New Mexico,[22][23] are typical supergene deposits formed from copper porphyries. The deposits inCochise County, Arizona, are found inCambrianquartzites and geologically younggranites and go down at least as deep as 54 meters (177 ft).[21]
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.[24]
Iran has been an important source of turquoise for at least 2,000 years. It was initially named byIranians "pērōzah" meaning "victory", and later theArabs called it "fayrūzah", which is pronounced in Modern Persian as "fīrūzeh". In Iranian architecture, the blue turquoise was used to cover thedomes ofpalaces because its intense blue colour was also a symbol of heaven on earth.[6][25][26][27]
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 inNishapur, the 2,012 m (6,601 ft) mountain peak ofAli-mersai nearMashhad, the capital ofKhorasan Province,Iran. Weathered and brokentrachyte is host to the turquoise, which is found bothin situ between layers of limonite and sandstone and amongst thescree at the mountain's base. These workings are the oldest known, together with those of theSinai Peninsula.[8] Iran also has turquoise mines inSemnan andKerman provinces.[6]
Since at least theFirst Dynasty (3000 BCE) inancient Egypt, and possibly before then, turquoise was used by theEgyptians and wasmined by them in theSinai Peninsula. This region was known as theCountry of Turquoise by the nativeMonitu. 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 historical perspective, areSerabit el-Khadim andWadi 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 deityHathor.
The turquoise is found insandstone that is, or was originally, overlain bybasalt. Copper and iron workings are present in the area. Large-scale turquoise mining is not profitable today, but the deposits are sporadically quarried byBedouin peoples using homemadegunpowder.[28] In the rainy winter months, miners face a risk fromflash flooding; even in the dry season, death from the collapse of the haphazardly exploited sandstone mine walls may occur. The colour of Sinai material is typically greener than that of 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 fine turquoise specimen fromLos Cerrillos, New Mexico, US, at theSmithsonian Museum. Cerrillos turquoise was widely used by Native Americans prior to the Spanish conquest.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
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"); highiron levels mean greens and yellows predominate, and a typically friable consistency in the turquoise's untreated state precludes use injewelry.
Arizona is currently the most important producer of turquoise by value.[8] 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 inGlobe 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. TheKingman 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. ThePhelps DodgeLavender Pit mine atBisbee 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 asnuggets, fracture fillings and inbreccias 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 inLander andEsmeralda counties. Most of the turquoise deposits in Nevada occur along a wide belt oftectonic activity that coincides with the state's zone ofthrust faulting. Itstrikes at a bearing of about 15° and extends from the northern part ofElko County, southward down to the California border southwest ofTonopah. 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 significantzinc andiron, which is the cause of the beautiful bright green to yellow-green shades. Some of the green to green-yellow shades may actually bevariscite orfaustite, 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 totalNevada 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 inMineral County, and several properties in the Royston andCandelaria areas of Esmerelda County.[29]
In 1912, the first deposit of distinct, single-crystal turquoise was discovered atLynch Station inCampbell County,Virginia. The crystals, forming adruse 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.[30]
In an attempt to recoup profits and meet demand, some American turquoise is treated orenhanced to a certain degree. These treatments include innocuous waxing and more controversial procedures, such as dyeing and impregnation (seeTreatments). There are 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.
Turquoise prehistoric artifacts (beads) are known since the fifth millennium BCE from sites in theEastern Rhodopes inBulgaria – the source for the raw material is possibly related to the nearbySpahievo lead–zinc ore field.[31] In Spain, turquoise has been found as a minor mineral in the variscite deposits exploited during prehistoric times in Palazuelos de las Cuevas (Zamora) and in Can Tintorer, Gavá (Barcelona).[32]
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, silicifiedlimestone ofYunxian andZhushan,Hubei province. Additionally,Marco Polo reported turquoise found in present-daySichuan. Most Chinese material is exported, but a few carvings worked in a manner similar tojade exist. InTibet, gem-quality deposits purportedly exist in the mountains ofDerge andNagari-Khorsum in the east and west of the region respectively.[33]
Trade in turquoise crafts, such as this freeform pendant dating from 1000 to 1040, is believed to have brought theAncestral Pueblo people of theChaco Canyon great wealth.Moche turquoise nose ornament.Larco Museum Collection, Lima, PeruBackswords, inlaid with turquoise. Russia, 17th centuryTurquoise mosaic mask ofXiuhtecuhtli, theAztec god of fire. The Aztecs differentiated turquoise based on quality:xihuitl, a more mundane version used for decoration such as in mosaics, andteoxihuitl, a special version embued with qualities ofTeotl and valued for its beauty.[34]The iconic gold burial mask ofTutankhamun, inlaid with turquoise,lapis lazuli,carnelian and colouredglass
The pastel shades of turquoise have endeared it to many great cultures of antiquity: it has adorned the rulers ofAncient Egypt, theAztecs (and possibly other Pre-ColumbianMesoamericans),Persia,Mesopotamia, theIndus Valley, and to some extent in ancientChina since at least theShang dynasty.[35] Despite being one of the oldest gems, probably first introduced toEurope (throughTurkey) with otherSilk Road novelties, turquoise did not become important as an ornamental stone in the West until the 14th century, following a decline in theRoman Catholic Church's influence which allowed the use of turquoise in secular jewellery. It was apparently unknown inIndia until theMughal period, and unknown inJapan 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 viewed turquoise as an embodiment of fire and gave it properties such as heat and smokiness. They inlaid turquoise, together withgold,quartz,malachite,jet,jade,coral, andshells, into provocative (and presumably ceremonial[clarification needed])mosaic objects such asmasks (some with ahuman skull as their base),knives, andshields. Naturalresins,bitumen andwax were used to bond the turquoise to the objects' base material; this was usuallywood, butbone and shell were also used. Like the Aztecs, thePueblo,Navajo andApache tribes cherished turquoise for its amuletic use; the latter tribe believe the stone to afford thearcher dead aim. In Navajo culture it is used for "a spiritual protection and blessing."[36] Among these peoples turquoise was used inmosaic inlay, in sculptural works, and was fashioned into toroidal beads and freeform pendants. TheAncestral Puebloans (Anasazi) of theChaco Canyon and surrounding region are believed to have prospered greatly from their production and trading of turquoise objects. The distinctivesilver 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 thede facto national stone for millennia, extensively used to decorate objects (fromturbans tobridles),mosques, and other important buildings both inside and out,[37] such as theMedresseh-i Shah Husein Mosque ofIsfahan. The Persian style and use of turquoise was later brought toIndia following the establishment of the Mughal Empire there, its influence seen in high puritygold jewellery (together withruby anddiamond) and in such buildings as theTaj Mahal. Persian turquoise was oftenengraved with devotional words inArabic 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 ofTibet andMongolia, 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.
TheAncient Egyptian use of turquoise stretches back as far as theFirst Dynasty and possibly earlier; however, probably the most well-known pieces incorporating the gem are those recovered fromTutankhamun's tomb, most notably thePharaoh's iconic burial mask which was liberally inlaid with the stone. It also adornedrings and great sweepingnecklaces calledpectorals. Set in gold, the gem was fashioned into beads, used as inlay, and often carved in ascarab motif, accompanied bycarnelian,lapis lazuli, and in later pieces, colouredglass. Turquoise, associated with the goddessHathor, was so liked by the Ancient Egyptians that it became (arguably[clarification needed]) the first gemstone to be imitated, the fair structure created by an artificial glazedceramic product known asfaience.
TheFrench conductedarchaeological 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, andart of the time. Turquoise, already favoured for its pastel shades since around 1810, was a staple ofEgyptian Revival pieces. In contemporary Western use, turquoise is most often encountered cuten 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 intofetishes, such as those crafted by theZuni. While strong sky blues remain superior in value, mottled green and yellowish material is popular withartisans.
The goddessHathor was associated with turquoise, as she was the patroness ofSerabit el-Khadim, where it was mined. Her titles included "Lady of Turquoise", "Mistress of Turquoise", and "Lady of Turquoise Country".[38]
In Western culture, turquoise is also the traditionalbirthstone for those born in the month of December. The turquoise is also a stone in the JewishHigh Priest'sbreastplate, described inExodus chapter 28. The stone is also considered sacred to the indigenousZuni andPueblo peoples of the American Southwest.[39][40] The pre-ColumbianAztec andMaya also considered it to be a valuable and culturally important stone.[41]
Some natural blue to blue-green materials, such as thisbotryoidalchrysocolla withdrusyquartz, 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 productfaience. Later glass andenamel were also used, and in modern times more sophisticatedporcelain,plastics, and various assembled, pressed, bonded, andsintered products (composed of various copper and aluminium compounds) have been developed: examples of the latter include "Viennese turquoise", made from precipitatedaluminium phosphate coloured bycopper oleate; and "neolith", a mixture ofbayerite andcopper(II) phosphate. Most of these products differ markedly from natural turquoise in both physical and chemical properties, but in 1972Pierre Gilson introduced one fairly close to a truesynthetic (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 dyedhowlite andmagnesite, both white in their natural states, and the former also having natural (and convincing) black veining similar to that of turquoise. Dyedchalcedony,jasper, andmarble is less common, and much less convincing. Other natural materials occasionally confused with or used in lieu of turquoise include:variscite andfaustite;[8]chrysocolla (especially when impregnatingquartz);lazulite;smithsonite;hemimorphite;wardite; and afossil bone ortooth calledodontolite or "bone turquoise", coloured blue naturally by the mineralvivianite. While rarely encountered today, odontolite was once mined in large quantities—specifically for its use as a substitute for turquoise—in southernFrance.
These fakes are detected bygemologists 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 be necessary; for example, the application of dilutedhydrochloric acid will cause thecarbonates odontolite and magnesite toeffervesce and howlite to turn green, while a heated probe may give rise to the pungent smell so indicative of plastic. Differences inspecific gravity,refractive index, light absorption (as evident in a material'sabsorption spectrum), and other physical and optical properties are also considered as means of separation.
Turquoise is treated to enhance both its colour and durability (increasedhardness and decreasedporosity). 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.
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.)
Since finer turquoise is often found as thin seams, it may beglued 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. Backing of turquoise is not widely known outside of theNative 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]
A proprietary process was created by electrical engineer and turquoise dealer James E. Zachery in the 1980s to improve the stability of medium to high-grade turquoise. The process can be applied in several ways: either through deep penetration on rough turquoise to decrease porosity, by shallow treatment of finished turquoise to enhance color, or both. The treatment can enhance color and improve the turquoise's ability to take a polish. Such treated turquoise can be distinguished in some cases from natural turquoise, without destruction, byenergy-dispersive X-ray spectroscopy, which can detect its elevated potassium levels. In some instances, such as with already high-quality, low-porosity turquoise that is treated only for porosity, the treatment is undetectable.[42][43]
The use ofPrussian 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,[44] especially since some dyes may fade or rub off on the wearer. Dyes have also been used to darken the veins of turquoise.
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 byepoxy andplastics (such aspolystyrene) andwater 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.[44]
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.
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" isartificial, 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".
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 strongsky torobin egg blue (in reference to the eggs of theAmerican robin).[33] 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 ormatrix 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;[2] 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 theSouthwest United States andFar East, but is not highly appreciated in theNear 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. Likecoral and other opaque gems, turquoise is commonly sold at a price according to its physical size in millimetres rather than weight.
Turquoise istreated 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 thelightwaxing oroiling 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 aphosphate mineral, turquoise is inherently fragile and sensitive to solvents;perfume and othercosmetics 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, includingsunscreen andhair 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.[citation needed]
^abSinkankas, John (1964).Mineralogy for amateurs. Princeton, N.J.: Van Nostrand. pp. 425–426.ISBN0442276249.{{cite book}}:ISBN / Date incompatibility (help)
^Farmani, Anousheh (2015)"Precious and semi-precious minerals and their roots in ancient Persian texts", Second International Congress of Science and Earth, Tehran[1]
^Zawush, Muhammad (1996).Mineralogy in Ancient Iran. tehran: Institute of Humanities and Cultural Studies. p. 156.ISBN9789644260315.
^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.
^Calvo Rebollar, Miguel (2015).Minerales y Minas de España. Vol. VII. Fosfatos, Arseniatos y Vanadatos [Minerals and mines of Spain] (in Spanish). Zaragoza, Spain: Escuela Técnica Superior de Ingenieros de Minas de Madrid. Fundación Gómez Pardo. pp. 347–348.ISBN978-84-95063-96-0.
^Bassett, Molly H. (2015).The fate of earthly things : Aztec gods and god-bodies (First ed.). Austin. p. 105.ISBN9780292760882.{{cite book}}: CS1 maint: location missing publisher (link)
^"China Exhibition". Washington, DC: National Gallery of Art. 1999. Archived fromthe original on 2006-09-30. Retrieved2006-09-23.
^Schwarzinger, Bettina; Schwarzinger, Clemens (May 2017). "Investigation of turquoise imitations and treatment with analytical pyrolysis and infrared spectroscopy".Journal of Analytical and Applied Pyrolysis.125:24–31.Bibcode:2017JAAP..125...24S.doi:10.1016/j.jaap.2017.05.002.
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Schadt, H. (1996).Goldsmith's Art: 5000 Years of Jewelry and Hollowware. Stuttgart & New York, NY: Arnoldsche Art Publisher.ISBN3-925369-54-6.
Webster, R. (2000).Gems: Their Sources, Descriptions and Identification (5th ed.). Great Britain: Butterworth-Heinemann. pp. 254–263.ISBN0-7506-1674-1.
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