Coral

Coral
Pillar coral, Dendrogyra cylindricus
Scientific classification
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Anthozoa Ehrenberg, 1831
Extant Subclasses and Orders
Alcyonaria    Alcyonacea    Helioporacea Zoantharia    Antipatharia    Corallimorpharia    Scleractinia    Zoanthidea [1][2]  See Anthozoa for details

Corals are marine invertebrates in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.
A coral "head" is a colony of myriad genetically identical polyps. Each polyp is a spineless animal typically only a few millimeters in diameter and a few centimeters in length. A set of tentacles surround a central mouth opening. An exoskeleton is excreted near the base. Over many generations, the colony thus creates a large skeleton that is characteristic of the species. Individual heads grow by asexual reproduction of polyps. Corals also breed sexually by spawning: polyps of the same species release gametes simultaneously over a period of one to several nights around a full moon.
Although some corals can catch small fish and plankton, using stinging cells on their tentacles, like those in sea anemone and jellyfish, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular algae that live within the coral's tissue called zooxanthella (also known as Symbiodinium). Such corals require sunlight and grow in clear, shallow water, typically at depths shallower than 60 metres (200 ft). Corals can be major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia. Other corals do not have associated algae and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 3,000 metres (9,800 ft).^[3] Examples live on the Darwin Mounds, north-west of Cape Wrath, Scotland. Corals have also been found off the coast of the U.S. in Washington State and the Aleutian Islands in Alaska.

Contents

• 1 Taxonomy
  • 1.1 Hermatypic corals
  • 1.2 Ahermatypic corals
  • 1.3 Perforate corals
• 2 Anatomy
  • 2.1 Colonial form
  • 2.2 Polyp
  • 2.3 Exoskeleton
  • 2.4 Tentacles
• 3 Ecology
  • 3.1 Feeding
  • 3.2 Intracellular symbionts
• 4 Reproduction
  • 4.1 Sexual
  • 4.2 Asexual
  • 4.3 Colony division
• 5 Reefs
• 6 Evolutionary history
• 7 Status
  • 7.1 Threats
  • 7.2 Protection
• 8 Relation to humans
  • 8.1 Jewelry
  • 8.2 Construction
  • 8.3 Climate research
  • 8.4 Aquaria
  • 8.5 Aquaculture
• 9 See also
• 10 Gallery
• 11 References
• 12 Further reading
• 13 External links

Taxonomy

A short tentacle plate coral in Papua New Guinea

Main articles: Anthozoa, Alcyonaria, and Zoantharia

Corals divide into two subclasses, depending on the number of tentacles or lines of symmetry, and a series of orders corresponding to their exoskeleton: nematocyst type and mitochondrial genetic analysis.^[1][2][4] Common coral typing crosses suborder/class boundaries.

Hermatypic corals

Further information: Scleractinia, Millepora, Tubipora, and Heliopora

Hermatypic corals in the subclass Scleractinia are stony corals that build reefs. They mostly obtain at least part of their energy requirements from zooxanthella (Symbiodinium), symbiotic photosynthetic microalgae. They secrete calcium carbonate to form a hard skeleton. Those having six or fewer lines of symmetry in their body structure are called hexacorallia or Zoantharia. This group includes reef-building corals (scleractinians), sea anemones and zoanthids. Hermatypic genera include Scleractinia, Millepora, Tubipora and Heliopora.^[5]
In the Caribbean alone, at least 50 species of uniquely structured hard coral exist. Well-known types include:

• Brain corals grow to 1.8 meters (5.9 ft) in width.
• Acropora and staghorn corals grow fast and large, and are important reef-builders. Staghorn coral displays large, antler-like branches, and grows in areas with strong surf.
• Pillar coral forms pillars which can grow to 3 meters (9.8 ft) in height.
• Leptopsammia, or rock coral, appears almost everywhere in the Caribbean.^[6]

Ahermatypic corals

Further information: Alcyonacea and Anthipatharia

Ahermatypic corals have no zooxanthella (Symbiodinium). They have eight tentacles and are also called octocorallia. They include corals in subclass Alcyonacesdasda, as well as some species in order Anthipatharia (black coral, Cirripathes, Antipathes).^[5] Ahermatypic corals, such as sea whips, sea feathers, and sea pens,^[6] are also known as soft corals. Unlike stony corals, they are flexible, undulating in the current, and often are perforated, with a lacy appearance. Their skeletons are proteinaceous, rather than calcareous. Soft corals are somewhat less plentiful (in the Caribbean, twenty species appear) than stony corals.

Perforate corals

Corals can be perforate or imperforate. Perforate corals have porous skeletons, which allows their polyps to connect with each other through the skeleton. Imperforate corals have hard solid skeletons.^[7][8]

Anatomy

Anatomy of a coral polyp

 

Flight through a µCT image stack of an Acropora coral from three views – note that the "arms" are mostly hollow. This coral had been hot glued into a stone and late grew over it.

 

Flight around a 3D object created from the data above.

The Muslim polymath Al-Biruni (d. 1048) classified sponges and corals as animals arguing that they respond to touch.^[9] Nevertheless, people believed coral to be a plant until the 18th century, when William Herschel used a microscope to establish that coral had the characteristic thin cell membranes of an animal.^[10]

Colonial form

The polyps interconnect by a complex and well-developed system of gastrovascular canals, allowing significant sharing of nutrients and symbiotes. In soft corals, these range in size from 50–500 micrometres (0.0020–0.020 in) in diameter, and allow transport of both metabolites and cellular components.^[11]

Close-up of Montastraea cavernosa polyps, the tentacles are clearly visible.

Polyp

While the coral head is the familiar visual form of a single organism, it is actually a group of many individual, yet genetically identical, multicellular organisms known as polyps. Polyps are usually a few millimeters in diameter, and are formed by a layer of outer epithelium and inner jellylike tissue known as the mesoglea. They are radially symmetrical, with tentacles surrounding a central mouth, the only opening to the stomach or coelenteron, through which food is ingested and waste expelled.

Exoskeleton

The stomach closes at the base of the polyp, where the epithelium produces an exoskeleton called the basal plate or calicle (L. small cup). The calicle is formed by a thickened calcareous ring (annular thickening) with six supporting radial ridges (as shown below). These structures grow vertically and project into the base of the polyp. When a polyp is physically stressed, its tentacles contract into the calyx so that virtually no part is exposed above the skeletal platform. This protects the organism from predators and the elements.^[12][13]
The polyp grows by extension of vertical calices which occasionally septate to form a new, higher, basal plate. Over many generations, this extension forms the large calcareous structures of corals and ultimately coral reefs.
Formation of the calcareous exoskeleton involves deposition of the mineral aragonite by the polyps from calcium and carbonate ions they acquire from seawater. The rate of deposition, while varying greatly across species and environmental conditions, can reach 10 g/m² per day (0.3 ounce/sq yd/day). This is light dependent, with night-time production 90% lower than that during the middle of the day.^[14]

Nematocyst discharge: A dormant nematocyst discharges response to nearby prey touching the cnidocil, the operculum flap opens, and its stinging apparatus fires the barb into the prey, leaving a hollow filament through which poisons are injected to immobilise the prey, then the tentacles manoeuvre the prey to the mouth.

Tentacles

Nematocysts at the tips of the calices are stinging cells that carry venom which they rapidly release in response to contact with another organism. The tentacles also bear a contractile band of epithelium called the pharynx. Jellyfish and sea anemones also carry nematocysts.

Ecology

Feeding

Polyps feed on a variety of small organisms, from microscopic demersal plankton to small fish. The polyp's tentacles immobilize or kill prey using their nematocysts (also known as 'cnidocysts'). The tentacles then contract to bring the prey into the stomach. Once the prey is digested, the stomach reopens, allowing the elimination of waste products and the beginning of the next hunting cycle. They can scavenge drifting organic molecules and dissolved organic molecules.^[15]:24

Intracellular symbionts

Many corals, as well as other cnidarian groups such as Aiptasia (a sea anemone) form a symbiotic relationship with a class of algae, zooxanthellae, of the genus Symbiodinium, a dinoflagellate.^[15]:24 Aiptasia, in a familiar pest among coral reef aquarium hobbyists, serves as a valuable model organism in the study of cnidarian-algal symbiosis. Typically, each polyp harbors one species of algae. Via photosynthesis, these provide energy for the coral, and aid in calcification.^[16] As much as 30% of the tissue of a polyp may be plant material.^[15]:23

Light and confocal images of Symbiodinium cells in hospite (living in a host cell) within scyphistomae of the jellyfish Cassiopea xamachana.

The algae benefit from a safe place to live and consume the polyp's carbon dioxide and nitrogenous waste. Due to the strain the algae can put on the polyp, stress on the coral often drives them to eject the algae. Mass ejections are known as coral bleaching, because the algae contribute to coral's brown coloration; other colors, however, are due to host coral pigments, such as green fluorescent proteins (GFPs). Ejection increases the polyp's chance of surviving short-term stress—they can regain algae, possibly of a different species at a later time. If the stressful conditions persist, the polyp eventually dies.^[17]

Reproduction

Corals can be both gonochoristic (unisexual) and hermaphroditic, each of which can reproduce sexually and asexually. Reproduction also allows coral to settle in new areas.

Sexual

Life cycles of broadcasters and brooders

Corals predominantly reproduce sexually. About 25% of hermatypic corals (stony corals) form single sex (gonochoristic) colonies, while the rest are hermaphroditic.^[18]

Broadcasters

About 75% of all hermatypic corals "broadcast spawn" by releasing gametes—eggs and sperm—into the water to spread offspring. The gametes fuse during fertilization to form a microscopic larva called a planula, typically pink and elliptical in shape. A typical coral colony forms several thousand larvae per year to overcome the odds against formation of a new colony.^[19]

A male star coral, Montastraea cavernosa, releases sperm into the water.

Synchronous spawning is very typical on the coral reef, and often, even when multiple species are present, all corals spawn on the same night. This synchrony is essential so male and female gametes can meet. Corals rely on environmental cues, varying from species to species, to determine the proper time to release gametes into the water. The cues involve temperature change, lunar cycle, day length, and possibly chemical signalling.^[18] Synchronous spawning may form hybrids and is perhaps involved in coral speciation.^[20] The immediate cue is most often sunset, which cues the release.^[18] The spawning event can be visually dramatic, clouding the usually clear water with gametes.

Brooders

Brooding species are most often ahermatypic (not reef-building) in areas of high current or wave action. Brooders release only sperm, which is negatively buoyant, sinking on to the waiting egg carriers who harbor unfertilized eggs for weeks. Synchronous spawning events sometimes occurs even with these species.^[18] After fertilization, the corals release planula that are ready to settle.^[16]

Planulae

Planulae exhibit positive phototaxis, swimming towards light to reach surface waters, where they drift and grow before descending to seek a hard surface to which they can attach and begin a new colony. They also exhibit positive sonotaxis, moving towards sounds that emanate from the reef and away from open water.^[21] High failure rates afflict many stages of this process, and even though millions of gametes are released by each colony, few new colonies form. The time from spawning to settling is usually two to three days, but can be up to two months.^[22] The larva grows into a polyp and eventually becomes a coral head by asexual budding and growth.

Asexual

Calices (basal plates) of Orbicella annularis showing multiplication by gemmation (small central calice) and division (large double calice)

The tabulate coral Aulopora (Devonian) showing initial budding from protocorallite

Within a coral head, the genetically identical polyps reproduce asexually, either via gemmation (budding) or by longitudinal or transversal division, both shown in the photo of Orbicella annularis.
Budding involves splitting a smaller polyp from an adult.^[19] As the new polyp grows, it forms its body parts. The distance between the new and adult polyps grows, and with it, the coenosarc (the common body of the colony; see coral anatomy). Budding can be:

• Intratentacular—from its oral discs, producing same-sized polyps within the ring of tentacles
• Extratentacular—from its base, producing a smaller polyp

Division forms two polyps each as large as the original. Longitudinal division begins when a polyp broadens and then divides its coelenteron, analogous to splitting a log along its length. The mouth also divides and new tentacles form. The two "new" polyps then generate their missing body parts and exoskeleton. Transversal division occurs when polyps and the exoskeleton divide transversally into two parts. This means one has the basal disc (bottom) and the other has the oral disc (top), similar to cutting the end off a log. The new polyps must separately generate the missing pieces.
Asexual reproduction has several benefits for these sessile colonial organisms:^[23]

• Cloning allows high reproduction rates, supporting rapid habitat exploitation.
• Modular growth allows biomass to increase without a corresponding decrease in surface-to-volume ratio.
• Modular growth delays senescence, by allowing the clone-type to survive the loss of one or more modules.
• New modules can replace dead modules, reducing clone-type mortality and preserving the colony's territory.
• Spreading the clone type to distant locations reduces clone-type mortality from localized threats.

Colony division

Whole colonies can reproduce asexually, forming two colonies with the same genotype.^{[citation needed]}

• Fission occurs in some corals, especially among the family Fungiidae, where the colony splits into two or more colonies during early developmental stages.
• Bailout occurs when a single polyp abandons the colony and settles on a different substrate to create a new colony.
• Fragmentation involves individuals broken from the colony during storms or other disruptions. The separated individuals can start new colonies.

Reefs

Locations of coral reefs

Main article: Coral reef

The hermatypic, stony corals are often found in coral reefs, large calcium carbonate structures generally found in shallow, tropical water. Reefs are built up from coral skeletons, and are held together by layers of calcium carbonate produced by coralline algae. Reefs are extremely diverse marine ecosystems hosting over 4,000 species of fish, massive numbers of cnidaria, mollusks, crustacea, and many other animals.^[24]

Evolutionary history

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Solitary rugose coral (Grewingkia) in three views; Ordovician, southeastern Indiana

[show]Left frame

[hide]Right frame

[show]Parallel view ()

[show]Cross-eye view ()

Horn coral fossil.

Although corals first appeared in the Cambrian period,^[25] some 542 million years ago, fossils are extremely rare until the Ordovician period, 100 million years later, when rugose and tabulate corals became widespread.
Tabulate corals occur in limestones and calcareous shales of the Ordovician and Silurian periods, and often form low cushions or branching masses alongside rugose corals. Their numbers began to decline during the middle of the Silurian period, and they became extinct at the end of the Permian period, 250 million years ago. The skeletons of tabulate corals are composed of a form of calcium carbonate known as calcite.
Rugose corals became dominant by the middle of the Silurian period, and became extinct early in the Triassic period. The rugose corals existed in solitary and colonial forms, and were also composed of calcite.
The scleractinian corals filled the niche vacated by the extinct rugose and tabulate species. Their fossils may be found in small numbers in rocks from the Triassic period, and became common in the Jurassic and later periods. Scleractinian skeletons are composed of a form of calcium carbonate known as aragonite.^[26] Although they are geologically younger than the tabulate and rugose corals, their aragonitic skeleton is less readily preserved, and their fossil record is less complete.

Timeline of the major coral fossil record and developments from 650 m.y.a. to present.[27][28]	edit

At certain times in the geological past, corals were very abundant. Like modern corals, these ancestors built reefs, some of which ended as great structures in sedimentary rocks.
Fossils of fellow reef-dwellers algae, sponges, and the remains of many echinoids, brachiopods, bivalves, gastropods, and trilobites appear along with coral fossils. This makes some corals useful index fossils that enabled geologists to date the rocks in which they are found. Coral fossils are not restricted to reef remnants, and many solitary fossils may be found elsewhere, such as Cyclocyathus, which occurs in England's Gault clay formation.
A Petoskey stone is a rock and a fossil, often pebble-shaped, that is composed of a fossilized coral, Hexagonaria percarinata. They are found predominantly in Michigan's Upper Peninsula, and the northwestern portion of Michigan's Lower Peninsula.

Status

Main article: Environmental issues with coral reefs

Threats

A healthy coral reef has a striking level of biodiversity in many forms of marine life.

Coral reefs are under stress around the world.^[29] In particular, coral mining, agricultural and urban runoff, pollution (organic and inorganic), overfishing, blast fishing, disease, and the digging of canals and access into islands and bays are localized threats to coral ecosystems. Broader threats are sea temperature rise, sea level rise and pH changes from ocean acidification, all associated with greenhouse gas emissions.^[30] In 1998, 16% of the world's reefs died as a result of increased water temperature.^[31]
General estimates show approximately 10% of the world's coral reefs are dead.^[32][33][34] About 60% of the world's reefs are at risk due to human-related activities.^[35] The threat to reef health is particularly strong in Southeast Asia, where 80% of reefs are endangered.^{[citation needed]} Over 50% of the world's coral reefs may be destroyed by 2030; as a result, most nations protect them through environmental laws.^[36]
In the Caribbean and tropical Pacific, direct contact between ~40–70% of common seaweeds and coral causes bleaching and death to the coral via transfer of lipid-soluble metabolites.^[37] Seaweed and algae proliferate given adequate nutrients and limited grazing by herbivores such as parrotfish.
Water temperature changes of more than 1–2 °C (1.8–3.6 °F) or salinity changes can kill some species of coral. Under such environmental stresses, corals expel their Symbiodinium; without them coral tissues reveal the white of their skeletons, an event known as coral bleaching.^[38]
Submarine springs found along the coast of Mexico's Yucatán Peninsula produce water with a naturally low pH (a measure of acidity) providing conditions similar to those expected to become widespread as the oceans absorb carbon dioxide^{[citation needed]}. Surveys discovered multiple species of live coral that appeared to tolerate the acidity. The colonies were small and patchily distributed, and had not formed structurally complex reefs such as those that compose the nearby Mesoamerican Barrier Reef System.^[39]

Protection

Main article: Coral reef protection

A diversity of corals

Marine Protected Areas (MPAs), Biosphere reserves, marine parks, national monuments world heritage status, fishery management and habitat protection can protect reefs from anthropogenic damage.^[40]

A section through a coral, dyed to determine growth rate

Many governments now prohibit removal of coral from reefs, and inform coastal residents about reef protection and ecology. While local action such as habitat restoration and herbivore protection can reduce local damage, the longer-term threats of acidification, temperature change and sea-level rise remain a challenge.^[30]
To eliminate destruction of corals in their indigenous regions, projects have been started to grow corals in non-tropical countries.^[41][42]

Relation to humans

Local economies near major coral reefs benefit from an abundance of fish and other marine creatures as a food source. Reefs also provide recreational scuba diving and snorkeling tourism. These activities can damage coral but international projects such as Green Fins that encourage dive and snorkel centres to follow a Code of Conduct has been proven to mitigate these risks.^[43]
In medicine, chemical compounds from corals are used for cancer, AIDS, pain, and other uses. Coral skeletons, e.g. Isididae are also used for bone grafting in humans.^[44]
Live coral is highly sought after for aquaria. Soft corals are easier to maintain in captivity than hard corals.^[45]

Jewelry

Main article: Coral (precious)

Corals' many colors give it appeal for necklaces and other jewelry. Intensely red coral is prized as a gemstone. Sometimes called fire coral, it is not the same as fire coral. Red coral is very rare because of overharvesting.^[46]

Construction

Tabulate coral (a syringoporid); Boone limestone (Lower Carboniferous) near Hiwasse, Arkansas, scale bar is 2.0 cm

Coral reefs on land provide lime for use as building blocks ("coral rag"). Coral rag is an important local building material in places such as the East African coast.^{[citation needed]}

Climate research

The annual growth bands in deep sea bamboo corals (Isididae) and others may be among the ocean's first organisms to display the effects of ocean acidification. They produce growth rings similar to those of trees, and can provide a view of changes in the condition in the deep sea over time.^[47] They allow geologists to construct year-by-year chronologies, a form of incremental dating, which underlie high-resolution records of past climatic and environmental changes using geochemical techniques.^[48]
Certain species form communities called microatolls, which are colonies whose top is dead and mostly above the water line, but whose perimeter is mostly submerged and alive. Average tide level limits their height. By analyzing the various growth morphologies, microatolls offer a low resolution record of sea level change. Fossilized microatolls can also be dated using radioactive carbon dating. Such methods can help to reconstruct Holocene sea levels.^[49]

Aquaria

Main article: Reef aquarium

This dragon-eye zoanthid is a popular source of color in reef tanks

The saltwater fishkeeping hobby has increasingly expanded, over recent years, to include reef tanks, fish tanks that include large amounts of live rock on which coral is allowed to grow and spread.^[50] These tanks are either kept in a natural-like state, with algae and a deep sand bed providing filtration,^[51] or as "show tanks", with the rock kept largely bare of the algae and microfauna that would normally populate it,^[52] in order to appear neat and clean.
The most popular kind of coral kept is soft coral, especially zoanthids and mushroom corals, which are especially easy to grow and propagate in a wide variety of conditions,^[53] because they originate in enclosed parts of reefs where water conditions vary and lighting may be less reliable and direct.^[54] More serious fishkeepers may keep small polyp stony coral, which is from open, brightly lit reef conditions and therefore much more demanding, while large polyp stony coral is a sort of compromise between the two.

Aquaculture

Main article: Aquaculture of coral

Coral aquaculture, also known as coral farming or coral gardening, is the cultivation of corals for commercial purposes or coral reef restoration. Aquaculture is showing promise as a potentially effective tool for restoring coral reefs, which have been declining around the world.^[55][56][57] The process bypasses the early growth stages of corals when they are most at risk of dying. Coral seeds are grown in nurseries then replanted on the reef.^[58] Coral is farmed by coral farmers who live locally to the reefs and farm for reef conservation or for income. It is also farmed by scientists for research, by businesses for the supply of the live and ornamental coral trade and by private aquarium hobbyists.

See also

• Bamboo coral
• Coral dermatitis
• Coral Triangle
• Organ pipe coral
• Sea Turtle Association of Japan, Kuroshima Research Station
• Brain Coral

Gallery

Further images: commons:Category:Coral reefs and commons:Category:Coral

• 

  Fungia sp. skeleton
  
• 

  Brain coral, Diploria labyrinthiformis
  
• 

  Polyps of Eusmilia fastigiata
  
• 

  Staghorn coral, Acropora
  
• 

  Orange cup coral, Balanophyllia elegans
  
• 

  Brain coral spawning
  
• 

  Brain coral releasing eggs
  
• 

  Fringing coral reef off the coast of Eilat, Israel.
  
• 

  Glass bottom boats and a Semi submarine which are used for coral viewing at Green Island (Queensland), Great Barrier Reef, Australia
  

Como são lapidados os diamantes?

O processo - que, além de aperfeiçoar o formato do diamante, serve para poli-lo - é feito de maneira artesanal. A qualidade da lapidação não apenas é fundamental para determinar o valor de uma jóia, como dá brilho e beleza à pedra.
Como o diamante é o material mais duro que se conhece na natureza, lapidá-lo não é moleza - sem contar o alto risco de estragar a caríssima pedra. "Quase sempre os lapidários a quem se confiam pedras maiores têm mais de 50 anos de idade. Isso porque leva muito tempo para aprender todos os macetes do processo", afirma o lapidário Renato Santos, presidente da Brasil Comércio de Diamantes.
Há duas formas de cortar o diamante bruto: na clivagem, o método mais comum, o diamante é partido com um rápido golpe. Em algumas pedras, porém, essa técnica não funciona. Usa-se, então, a serragem, processo longo e tedioso, feito com uma serra elétrica rotatória ou, mais recentemente, com raios laser.
Depois do corte, vem a etapa do bloqueamento, em que o diamante é raspado em outro até que se aproxime do formato desejado. As facetas (como são chamadas as várias pequenas faces de um diamante) são feitas na etapa seguinte, chamada de abrilhantamento. A pedra é encaixada na ponta de uma vareta chamada dop e pressionada contra um disco giratório forrado de pó de diamante. O processo lembra um pouco o de uma agulha riscando um disco de vinil na vitrola.
Em geral, os brilhantes pequenos são lapidados em um único dia. Já nas pedras grandes (acima de 20 gramas) esse trabalho pode levar até mais de um ano!

Turquoise

Turquoise
Turquoise, tumble finished, one inch (25 mm) long.
General
Category	Phosphate minerals
Formula (repeating unit)	CuAl6(PO4)4(OH)8·4H2O
Strunz classification	08.DD.15
Identification
Colour	Blue, blue-green, green
Crystal habit	Massive, nodular
Crystal system	Triclinic
Cleavage	Good to perfect_usually N/A
Fracture	Conchoidal
Mohs scale hardness	5–7
Lustre	Waxy to subvitreous
Streak	Bluish white
Specific gravity	2.6–2.9
Optical properties	Biaxial (+)
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 hydrous phosphate of copper and aluminium, with the chemical formula CuAl₆(PO
4)₄(OH)₈·4H
2O. It is rare and valuable in finer grades and has been prized as a gem and ornamental stone for thousands of years owing to its unique hue. In recent times, turquoise, like most other opaque gems, has been devalued by the introduction of treatments, imitations, and synthetics onto the market.
The substance has been known by many names, but the word turquoise, which dates to the 16th century, is derived from an Old French word for "Turkish", because the mineral was first brought to Europe from Turkey, from the mines in historical Khorasan Province of Iran.^[2][3][4][5] Pliny the Elder referred to the mineral as callais, the Iranians named it "phirouzeh" and the Aztecs knew it as Teoxihuitl.^[4]

Contents

• 1 Properties of turquoise
• 2 Formation
• 3 Occurrence
  • 3.1 Iran
  • 3.2 Sinai
  • 3.3 United States
  • 3.4 Other sources
• 4 History of its use
• 5 Culture
• 6 Imitations
• 7 Treatments
  • 7.1 Waxing and oiling
  • 7.2 Stabilization
  • 7.3 Dyeing
  • 7.4 Reconstitution
  • 7.5 Backing
• 8 Valuation and care
• 9 Gallery
• 10 See also
• 11 References
• 12 Further reading

Properties of turquoise

Even the finest of turquoise is fracturable, reaching a maximum 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. Its crystal system is proven to be triclinic via X-ray diffraction testing. With lower hardness comes lower specific gravity (2.60–2.90) and greater porosity: These properties are dependent on grain size. The lustre of turquoise is typically waxy to subvitreous, and transparency is usually opaque, but may be semitranslucent in thin sections. Colour is as variable as the mineral's other properties, ranging from white to a powder blue to a sky blue, and from a blue-green to a yellowish green. The blue is attributed to idiochromatic copper while the green may be the result of either iron impurities (replacing aluminium) or dehydration.
The refractive index (as measured by sodium light, 589.3 nm) of turquoise is approximately 1.61 or 1.62; this is a mean value seen as a single reading on a gemmological refractometer, owing to the almost invariably polycrystalline nature of turquoise. A reading of 1.61–1.65 (birefringence 0.040, biaxial positive) has been taken from rare single crystals. An absorption spectrum may also be obtained with a hand-held spectroscope, revealing a line at 432 nanometres and a weak band at 460 nanometres (this is best seen with strong reflected light). Under longwave ultraviolet light, turquoise may occasionally fluoresce green, yellow or bright blue; it is inert under shortwave ultraviolet and X-rays.
Turquoise is insoluble in all but heated hydrochloric acid. Its streak is a pale bluish white and its fracture is conchoidal, leaving a waxy lustre. Despite its low hardness relative to other gems, turquoise takes a good polish. Turquoise may also be peppered with flecks of pyrite or interspersed with dark, spidery limonite veining.

Formation

As a secondary mineral, turquoise apparently 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 American southwest 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 ft), although it does occur along deeper fracture zones where secondary solutions have greater penetration or the depth to the water table is greater.
Although the features of turquoise occurrences are consistent with a secondary or supergene origin, some sources refer to a hypogene origin. The hypogene hypothesis holds that the aqueous solutions originate at significant depth, from hydrothermal processes. Initially at high temperature, these solutions rise upward to surface layers, interacting with, and leaching essential elements from pre-existing minerals in the process. As the solutions cool, turquoise precipitates, lining cavities and fractures within the surrounding rock. This hypogene process is applicable to the original copper sulfide deposition; however, it is difficult to account for the many features of turquoise occurrences by a hypogene process. That said, there are reports of two phase fluid inclusions within turquoise grains that give elevated homogenization temperatures of 90 to 190 °C that require explanation.
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 Mineral Park, Arizona

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

Cutting and grinding turquoise in Nishapur, Iran, 1973

Iran

For at least 2,000 years, Iran, known before as Persia, has remained an important source of turquoise which was named by Iranians initially "pirouzeh" meaning "victory" and later after Arab invasion "firouzeh".^{[citation needed]} In Iranian architecture, the blue turquoise was used to cover the domes of the Iranian palaces because its intense blue colour was also a symbol of heaven on earth.^{[citation needed]}
This deposit, which is blue naturally, and turns green when heated due to dehydration, is restricted to a mine-riddled region in Nishapur, the 2,012-metre (6,601 ft) mountain peak of Ali-mersai, which is tens of kilometers from Mashhad, the capital of Khorasan province, Iran. A weathered and broken trachyte is host to the turquoise, which is found both in situ between layers of limonite and sandstone, and amongst the scree at the mountain's base. These workings, together with those of the Sinai Peninsula, are the oldest known.^[5]

Sinai

Since at least the First Dynasty (3000 BCE), and possibly before then, turquoise was used by the Egyptians and was mined by them in the Sinai Peninsula, called "Country of Turquoise" by the native Monitu. There are six mines in the region, all on the southwest coast of the peninsula, covering an area of some 650 square kilometres (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 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. 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.

United States

A selection of Ancestral Puebloan (Anasazi) turquoise and orange argillite inlay pieces from Chaco Canyon (dated ca. 1020–1140 CE) show the typical colour range and mottling of American turquoise.

Bisbee turquoise commonly has a hard chocolate brown coloured matrix.

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) 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, and the Kingman Mine that operates alongside a copper mine outside of the city. Other active mines include the Blue Bird mine, Castle Dome, and Ithaca Peak. The mines at Morenci, Bisbee, 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 about N15°E 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, the total Nevada turquoise production since the 1870s has been estimated at more than 600 tons, including nearly 400 tons 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.^[6]

Untreated turquoise, Nevada USA. Rough nuggets from the McGinness Mine, Austin; Blue and green cabochons showing spiderweb, Bunker Hill Mine, Royston

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.^[7]
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

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.^[8]
Other notable localities include: Afghanistan; Australia (Victoria and Queensland); north India; northern Chile (Chuquicamata); Cornwall; Saxony; Silesia; and Turkestan.

History of its use

Trade in turquoise crafts, such as this freeform pendant dating from 1000–1040 CE, is believed to have brought the Ancestral Puebloans of the Chaco Canyon great wealth.

Moche turquoise nose ornament. Larco Museum Collection. Lima-Peru

Backswords, inlaid with turquoise. Russia, 17th century.

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.

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.^[9] 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 circa 1880 as a result of European influences.
In Persia, turquoise was the de facto national stone for millennia, extensively used to decorate objects (from turbans to bridles), mosques, and other important buildings both inside and out, such as the Medresseh-I Shah Husein Mosque of Isfahan. The Persian style and use of turquoise was later brought to India following the establishment of the Mughal Empire there, its influence seen in high 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 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 c. 1810, was a staple of Egyptian Revival pieces. In contemporary Western use, turquoise is most often encountered cut en cabochon in silver rings, bracelets, often in the Native American style, or as tumbled or roughly hewn beads in chunky necklaces. Lesser material may be carved into fetishes, such as those crafted by the Zuni. While strong sky blues remain superior in value, mottled green and yellowish material is popular with artisans. In Western culture, turquoise is also the traditional birthstone for those born in the month of December. The turquoise is also a stone in the Jewish High Priest's breastplate, described in Exodus 28.

Culture

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. It really does have the right to be called a 'gemstone of the peoples'. 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.

Imitations

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

The 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 ceramics, porcelain, plastics, and various assembled, pressed, bonded, and sintered products (composed of various copper and aluminium compounds) have been developed: examples of the latter include "Viennese turquoise", made from precipitated aluminium phosphate coloured by copper oleate; and "neolith", a mixture of bayerite and copper phosphate. Most of these products differ markedly from natural turquoise in both physical and chemical properties, but in 1972 Pierre Gilson introduced one fairly close to a true synthetic (it does differ in chemical composition owing to a binder used, meaning it is best described as a simulant rather than a synthetic). Gilson turquoise is made in both a uniform colour and with black "spiderweb matrix" veining not unlike the natural Nevada material.
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 gemmologists 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 the Madan village of Khorasan.

Turquoise is treated to enhance both its colour and durability (i.e., increased hardness and decreased porosity). As is so often the case with any precious stones, full disclosure about treatment is frequently not given. It is therefore left to gemologists to detect these treatments in suspect stones using a variety of testing methods—some of which are necessarily destructive. For example, the use of a heated probe applied to an inconspicuous spot will reveal oil, wax, or plastic treatment with certainty.

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.^[10]
The epoxy binding technique was first developed in the 1950s and has been attributed to Colbaugh Processing of Arizona, a company that still operates today. The majority of American material is now treated in this manner although it is a costly process requiring many months to complete. Without such impregnation, most American mining operations would be unprofitable.

Dyeing

The use of Prussian blue and other dyes (often in conjunction with bonding treatments) to "enhance"—that is, make uniform or completely change—colour is regarded as fraudulent by some purists,^[10] 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 to form a solid mass. Very often the material sold as "reconstituted" turquoise is artificial, with little or no natural stone, and may have foreign filler material added to it.

Backing

Since finer turquoise is often found as thin seams, it may be glued to a base of stronger foreign material as a means of reinforcement. These 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 cabs 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

American Robin nest and eggs

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's egg" blue (in reference to the eggs of the American Robin).^[8] Whatever the colour, turquoise should not be excessively soft or chalky; even if treated, such lesser material (to which most turquoise belongs) is liable to fade or discolour over time and will not hold up to normal use in jewellery.
The mother rock or matrix in which turquoise is found can often be seen as splotches or a network of brown or black veins running through the stone in a netted pattern; this veining may add value to the stone if the result is 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 build up 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.

Gallery

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  Turquoise tiles on the facade of the Dome of the Rock in Jerusalem.
  
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  Cupula of the famous Tilla Kari Mosque in Samarkand, Uzbekistan.
  
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  The portal of the St. Petersburg mosque.
  
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  The turquoise domes of the Qol-Şärif mosque in Kazan.
  
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  The Madrassa of Ulugh Beg at the Registan Mosque Complex in Samarkand.
  
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  The Abbasid Ivan of the Goharshad mosque in Mashad, Iran.
  
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  The dome and minaret of the Isfahan Royal Mosque in Isfahan, Iran.
  
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  Minaret of the Koutoubia Mosque in Marrakesh, Morocco.
  
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  The mausoleum of the Persian mystic Jalal ad-Din Muhammad Rumi at Konya, Turkey.
  
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  The Rawze-e-Sharif at Mazar-i-Sharif in Afghanistan.