These bags weigh on
average 10-15 carats, which is 2-3 grams. A bag will have 10 pieces per bag.
The nuggets measure on average 5-8 mm per stone.
I offer a shipping discount for
customers who combine their payments for multiple purchases into one payment!
The discount is regular shipping price for the first item and just 50 cents for
each additional item! To be sure you get your shipping discount just make sure
all the items you want to purchase are in your cart. Auctions you win are added
to your cart automatically. For any "buy it now" items or second
chance offers, be sure to click the "add to cart" button, NOT the
"buy it now" button. Once all of your items are in your cart just pay
for them from your cart and the combined shipping discount should be applied
automatically.
I offer a money back guarantee on
every item I sell. If you are not 100% happy with your purchase just send me a
message to let me know and I will buy back the item for your full purchase
price.
Hi there, I am selling this really Lot
of 10 Turquoise Nuggets natural Mineral Specimens. These are from an old
collection of mine. I purchased it at an
estate of an old miner. It comes from the Kingman Mine in Arizona , USA area which was famous
for its turquoise. Don't pass up the chance to purchase these rare specimens! These
have the most Amazing Teal Blue color, or maybe the colors can be described as
a robin's egg blue, they are just stunning. If you have any questions, do not
hesitate to ask me. Thanks so much for visiting my store and have a great
day:>)
The following is information about this
from wikipedia:
Turquoise
From Wikipedia, the free encyclopedia
Jump to navigationJump to search
For other uses, see Turquoise
(disambiguation).
Turquoise
General
Category Phosphate
minerals
Formula
(repeating
unit) CuAl6(PO4)4(OH)8·4H2O
Strunz classification 8.DD.15
Crystal
system Triclinic
Crystal class Pinacoidal (1)
(same H–M symbol)
Identification
Colour Turquoise,
blue, blue-green, green
Crystal habit Massive, nodular
Cleavage Good
to perfect_usually N/A
Fracture Conchoidal
Mohs scale
hardness 5–6
Luster 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 hydrated phosphate of copper and aluminium, with the chemical
formula CuAl6(PO4)4(OH)8·4H2O. It is rare and valuable in finer grades and has
been prized as a gemstone and ornamental stone for thousands of years owing to
its unique hue. In recent times, turquoise has been devalued, like most other
opaque gems, by the introduction onto the market of treatments, imitations and
synthetics.
The gemstone has been known by many
names. Pliny the Elder referred to the mineral as callais (from Ancient Greek
κάλαϊς) and the Aztecs knew it as chalchihuitl.[4] The word turquoise dates to
the 17th century and is derived from the French turquois for
"Turkish" because the mineral was first brought to Europe through
Turkey, from mines in the historical Khorasan Province of Persia.[2][3][4][5]
Contents
1 Properties
2 Formation
3 Occurrence
3.2 Sinai
3.3 United
States
3.4 Other
sources
4 History
of use
5 Cultural
associations
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 See
also
10 References
11 Further
reading
12 External
links
Properties
The finest of turquoise reaches a
maximum Mohs hardness of just under 6, or slightly more than window glass.[2]
Characteristically a cryptocrystalline mineral, turquoise almost never forms
single crystals, and all of its properties are highly variable. X-ray
diffraction testing shows its crystal system to be triclinic.[3][6] With lower
hardness comes lower specific gravity (2.60–2.90)[3] and greater porosity;
these properties are dependent on grain size. The lustre of turquoise is
typically waxy to subvitreous, and its transparency is usually opaque, but may
be semitranslucent in thin sections. Colour is as variable as the mineral's
other properties, ranging from white to a powder blue to a sky blue, and from a
blue-green to a yellowish green. The blue is attributed to idiochromatic copper
while the green may be the result of either iron impurities (replacing
aluminium) or dehydration.
The refractive index of turquoise (as
measured by sodium light, 589.3 nm) is approximately 1.61 or 1.62; this is a
mean value seen as a single reading on a gemological refractometer, owing to
the almost invariably polycrystalline nature of turquoise. A reading of
1.61–1.65 (birefringence 0.040, biaxial positive) has been taken from rare
single crystals. An absorption spectrum may also be obtained with a hand-held
spectroscope, revealing a line at 432 nm and a weak band at 460 nm (this is
best seen with strong reflected light). Under longwave ultraviolet light,
turquoise may occasionally fluoresce green, yellow or bright blue; it is inert
under shortwave ultraviolet and X-rays.
Turquoise is insoluble in all but
heated hydrochloric acid. Its streak is a pale bluish white and its fracture is
conchoidal,[3] leaving a waxy lustre. Despite its low hardness relative to
other gems, turquoise takes a good polish. Turquoise may also be peppered with
flecks of pyrite or interspersed with dark, spidery limonite veining.
Formation
"Big Blue", a large turquoise
specimen from the copper mine at Cananea, Sonora, Mexico
As a secondary mineral, turquoise forms
by the action of percolating acidic aqueous solutions during the weathering and
oxidation of pre-existing minerals. For example, the copper may come from
primary copper sulfides such as chalcopyrite or from the secondary carbonates
malachite or azurite; the aluminium may derive from feldspar; and the
phosphorus from apatite. Climate factors appear to play an important role as
turquoise is typically found in arid regions, filling or encrusting cavities
and fractures in typically highly altered volcanic rocks, often with associated
limonite and other iron oxides. In the Southwestern United States turquoise is
almost invariably associated with the weathering products of copper sulfide
deposits in or around potassium-feldspar-bearing porphyritic intrusives. In
some occurrences alunite, potassium aluminium sulfate, is a prominent secondary
mineral. Typically turquoise mineralization is restricted to a relatively
shallow depth of less than 20 metres (66 feet), although it does occur along
deeper fracture zones where secondary solutions have greater penetration or the
depth to the water table is greater.
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, US Turquoise was among the first
gems to be mined, and many historic sites have been depleted, though some are
still worked to this day. These are all small-scale operations, often seasonal
owing to the limited scope and remoteness of the deposits. Most are worked by
hand with little or no mechanization. However, turquoise is often recovered as
a byproduct of large-scale copper mining operations, especially in the United States.
Sinai
Since at least the First Dynasty (3000
BCE) in ancient Egypt, and possibly before then, turquoise was used by the
Egyptians and was mined by them in the Sinai Peninsula. This region was known
as the Country of Turquoise by the native Monitu. There are six mines in the
peninsula, all on its southwest coast, covering an area of some 650 km2 (250 sq
mi). The two most important of these mines, from a historic perspective, are
Serabit el-Khadim and Wadi Maghareh, believed to be among the oldest of known
mines. The former mine is situated about 4 kilometres from an ancient temple
dedicated to the deity Hathor.
The turquoise is found in sandstone
that is, or was originally, overlain by basalt. Copper and iron workings are
present in the area. Large-scale turquoise mining is not profitable today, but
the deposits are sporadically quarried by Bedouin peoples using homemade
gunpowder.[citation needed] In the rainy winter months, miners face a risk from
flash flooding; even in the dry season, death from the collapse of the
haphazardly exploited sandstone mine walls is not unheard of. The colour of
Sinai material is typically greener than Persian material, but is thought to be
stable and fairly durable. Often referred to as "Egyptian turquoise",
Sinai material is typically the most translucent, and under magnification its
surface structure is revealed to be peppered with dark blue discs not seen in
material from other localities.
A selection of Ancestral Pueblo
(Anasazi) turquoise and orange argillite inlay pieces from Chaco Canyon, New
Mexico, US (dated c. 1020–1140) show the typical colour range and mottling of
American turquoise. Some likely came from Los Cerrillos.
United States
A fine turquoise specimen from Los
Cerrillos, New Mexico, US, at the Smithsonian Museum. Cerrillos turquoise was
widely used by Native Americans prior to the Spanish conquest.
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 Southwest United States is a
significant source of turquoise; Arizona, California (San Bernardino, Imperial,
Inyo counties), Colorado (Conejos, El Paso, Lake, Saguache counties), New
Mexico (Eddy, Grant, Otero, Santa Fe counties) and Nevada (Clark, Elko,
Esmeralda County, Eureka, Lander, Mineral County and Nye counties) are (or
were) especially rich. The deposits of California and New Mexico were mined by
pre-Columbian Native Americans using stone tools, some local and some from as
far away as central Mexico. Cerrillos, New Mexico is thought to be the location
of the oldest mines; prior to the 1920s, the state was the country's largest
producer; it is more or less exhausted today. Only one mine in California,
located at Apache Canyon, operates at a commercial capacity today.
The turquoise occurs as vein or seam
fillings, and as compact nuggets; these are mostly small in size. While quite
fine material is sometimes found, rivalling Persian material in both colour and
durability, most American turquoise is of a low grade (called "chalk
turquoise"); high iron levels mean greens and yellows predominate, and a
typically friable consistency in the turquoise's untreated state precludes use
in jewellery.
Arizona is currently the most important
producer of turquoise by value.[5] Several mines exist in the state, two of
them famous for their unique colour and quality and considered the best in the
industry: the Sleeping Beauty Mine in Globe ceased turquoise mining in August
2012. The mine chose to send all ore to the crusher and to concentrate on
copper production due to the rising price of copper on the world market. The
price of natural untreated Sleeping Beauty turquoise has risen dramatically
since the mine's closing. The Kingman Mine as of 2015 still operates alongside
a copper mine outside of the city. Other mines include the Blue Bird mine,
Castle Dome, and Ithaca Peak, but they are mostly inactive due to the high cost
of operations and federal regulations The Phelps Dodge Lavender Pit mine at
Bisbee ceased operations in 1974 and never had a turquoise contractor. All
Bisbee turquoise was "lunch pail" mined. It came out of the copper
ore mine in miners' lunch pails. Morenci and Turquoise Peak are either inactive
or depleted.
Nevada is the country's other major producer,
with more than 120 mines which have yielded significant quantities of
turquoise. Unlike elsewhere in the US, most Nevada mines have been worked
primarily for their gem turquoise and very little has been recovered as a
byproduct of other mining operations. Nevada turquoise is found as nuggets,
fracture fillings and in breccias as the cement filling interstices between
fragments. Because of the geology of the Nevada deposits, a majority of the
material produced is hard and dense, being of sufficient quality that no
treatment or enhancement is required. While nearly every county in the state
has yielded some turquoise, the chief producers are in Lander and Esmeralda
counties. Most of the turquoise deposits in Nevada occur along a wide belt of
tectonic activity that coincides with the state's zone of thrust faulting. It
strikes 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.[7]
In 1912, the first deposit of distinct,
single-crystal turquoise was discovered in Lynch Station, Campbell County,
Virginia. The crystals, forming a druse over the mother rock, are very small; 1
mm (0.04 in) is considered large. Until the 1980s Virginia was widely thought
to be the only source of distinct crystals; there are now at least 27 other
localities.[citation needed]
In an attempt to recoup profits and
meet demand, some American turquoise is treated or enhanced to a certain
degree. These treatments include innocuous waxing and more controversial
procedures, such as dyeing and impregnation (see Treatments). There are
however, some American mines which produce materials of high enough quality
that no treatment or alterations are required. Any such treatments which have
been performed should be disclosed to the buyer on sale of the material.
Other sources
Turquoise prehistoric artefacts (beads)
are known since the fifth millennium BCE from sites in the Eastern Rhodopes in
Bulgaria - the source for the raw material is possibly related to the nearby
Spahievo Ph-Zn ore field.[8]
China has been a minor source of
turquoise for 3,000 years or more. Gem-quality material, in the form of compact
nodules, is found in the fractured, silicified limestone of Yunxian and
Zhushan, Hubei province. Additionally, Marco Polo reported turquoise found in
present-day Sichuan. Most Chinese material is exported, but a few carvings
worked in a manner similar to jade exist. In Tibet, gem-quality deposits
purportedly exist in the mountains of Derge and Nagari-Khorsum in the east and
west of the region respectively.[9]
Other notable localities include:
Afghanistan; Australia (Victoria and Queensland); north India; northern Chile
(Chuquicamata); Cornwall; Saxony; Silesia; and Turkestan.
History of use
Trade in turquoise crafts, such as this
freeform pendant dating from 1000–1040, is believed to have brought the
Ancestral Puebloans of the Chaco Canyon great wealth.
The pastel shades of turquoise have
endeared it to many great cultures of antiquity: it has adorned the rulers of
Ancient Egypt, the Aztecs (and possibly other Pre-Columbian Mesoamericans),
Persia, Mesopotamia, the Indus Valley, and to some extent in ancient China
since at least the Shang Dynasty.[10] Despite being one of the oldest gems,
probably first introduced to Europe (through Turkey) with other Silk Road
novelties, turquoise did not become important as an ornamental stone in the
West until the 14th century, following a decline in the Roman Catholic Church's
influence which allowed the use of turquoise in secular jewellery. It was
apparently unknown in India until the Mughal period, and unknown in Japan until
the 18th century. A common belief shared by many of these civilizations held
that turquoise possessed certain prophylactic qualities; it was thought to
change colour with the wearer's health and protect him or her from untoward
forces.
The Aztecs inlaid turquoise, together
with gold, quartz, malachite, jet, jade, coral, and shells, into provocative
(and presumably ceremonial) mosaic objects such as masks (some with a human
skull as their base), knives, and shields. Natural resins, bitumen and wax were
used to bond the turquoise to the objects' base material; this was usually
wood, but bone and shell were also used. Like the Aztecs, the Pueblo, Navajo
and Apache tribes cherished turquoise for its amuletic use; the latter tribe
believe the stone to afford the archer dead aim. Among these peoples turquoise
was used in mosaic inlay, in sculptural works, and was fashioned into toroidal
beads and freeform pendants. The Ancestral Puebloans (Anasazi) of the Chaco
Canyon and surrounding region are believed to have prospered greatly from their
production and trading of turquoise objects. The distinctive silver jewellery
produced by the Navajo and other Southwestern Native American tribes today is a
rather modern development, thought to date from 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,[citation needed] such as the Medresseh-I Shah Husein Mosque of Isfahan.
The Persian style and use of turquoise was later brought to India following the
establishment of the Mughal Empire there, its influence seen in high purity
gold jewellery (together with ruby and diamond) and in such buildings as the
Taj Mahal. Persian turquoise was often engraved with devotional words in Arabic
script which was then inlaid with gold.
Cabochons of imported turquoise, along
with coral, was (and still is) used extensively in the silver and gold
jewellery of Tibet and Mongolia, where a greener hue is said to be preferred.
Most of the pieces made today, with turquoise usually roughly polished into
irregular cabochons set simply in silver, are meant for inexpensive export to
Western markets and are probably not accurate representations of the original
style.
The Egyptian use of turquoise stretches
back as far as the First Dynasty and possibly earlier; however, probably the
most well-known pieces incorporating the gem are those recovered from
Tutankhamun's tomb, most notably the Pharaoh's iconic burial mask which was
liberally inlaid with the stone. It also adorned rings and great sweeping
necklaces called pectorals. Set in gold, the gem was fashioned into beads, used
as inlay, and often carved in a scarab motif, accompanied by carnelian, lapis
lazuli, and in later pieces, coloured glass. Turquoise, associated with the
goddess Hathor, was so liked by the Ancient Egyptians that it became (arguably)
the first gemstone to be imitated, the fair structure created by an artificial
glazed ceramic product known as faience.
The French conducted archaeological
excavations of Egypt from the mid-19th century through the early 20th. These
excavations, including that of Tutankhamun's tomb, created great public
interest in the western world, subsequently influencing jewellery,
architecture, and art of the time. Turquoise, already favoured for its pastel
shades since around 1810, was a staple of Egyptian Revival pieces. In
contemporary Western use, turquoise is most often encountered cut en cabochon in
silver rings, bracelets, often in the Native American style, or as tumbled or
roughly hewn beads in chunky necklaces. Lesser material may be carved into
fetishes, such as those crafted by the Zuni. While strong sky blues remain
superior in value, mottled green and yellowish material is popular with
artisans.
Cultural associations
In many cultures of the Old and New
Worlds, this gemstone has been esteemed for thousands of years as a holy stone,
a bringer of good fortune or a talisman. The oldest evidence for this claim was
found in Ancient Egypt, where grave furnishings with turquoise inlay were
discovered, dating from approximately 3000 BCE. In the ancient Persian Empire,
the sky-blue gemstones were earlier worn round the neck or wrist as protection
against unnatural death. If they changed colour, the wearer was thought to have
reason to fear the approach of doom. Meanwhile, it has been discovered that the
turquoise certainly can change colour, but that this is not necessarily a sign
of impending danger. The change can be caused by the light, or by a chemical
reaction brought about by cosmetics, dust or the acidity of the skin.[11]
The goddess Hathor was associated with
turquoise, as she was the patroness of Serabit el-Khadim, where it was mined.
Her titles included "Lady of Turquoise", "Mistress of
Turquoise", and "Lady of Turquoise Country".[12]
In Western culture, turquoise is also
the traditional birthstone for those born in the month of December. The
turquoise is also a stone in the Jewish High Priest's breastplate, described in
Exodus 28. The stone is also considered sacred to the indigenous peoples of the
Southwestern United States[13] Zuni and Pueblo peoples of the American
Southwest,[14] The pre-Columbian Aztec and Maya also considered it to be a valuable
and culturally important stone.[15]
Imitations
Some natural blue to blue-green
materials, such as this botryoidal chrysocolla with drusy quartz, are
occasionally confused with or used to imitate turquoise.
The Egyptians were the first to produce
an artificial imitation of turquoise, in the glazed earthenware product
faience. Later glass and enamel were also used, and in modern times more
sophisticated porcelain, plastics, and various assembled, pressed, bonded, and
sintered products (composed of various copper and aluminium compounds) have
been developed: examples of the latter include "Viennese turquoise",
made from precipitated aluminium phosphate coloured by copper oleate; and
"neolith", a mixture of bayerite and copper phosphate. Most of these
products differ markedly from natural turquoise in both physical and chemical
properties, but in 1972 Pierre Gilson introduced one fairly close to a true
synthetic (it does differ in chemical composition owing to a binder used,
meaning it is best described as a simulant rather than a synthetic). Gilson
turquoise is made in both a uniform colour and with black "spiderweb
matrix" veining not unlike the natural Nevada material.
The most common imitation of turquoise
encountered today is dyed howlite and magnesite, both white in their natural
states, and the former also having natural (and convincing) black veining
similar to that of turquoise. Dyed chalcedony, jasper, and marble is less
common, and much less convincing. Other natural materials occasionally confused
with or used in lieu of turquoise include: variscite and faustite;[5]
chrysocolla (especially when impregnating quartz); lazulite; smithsonite;
hemimorphite; wardite; and a fossil bone or tooth called odontolite or
"bone turquoise", coloured blue naturally by the mineral vivianite.
While rarely encountered today, odontolite was once mined in large
quantities—specifically for its use as a substitute for turquoise—in southern
France.
These fakes are detected by gemologists
using a number of tests, relying primarily on non-destructive, close
examination of surface structure under magnification; a featureless, pale blue
background peppered by flecks or spots of whitish material is the typical
surface appearance of natural turquoise, while manufactured imitations will
appear radically different in both colour (usually a uniform dark blue) and
texture (usually granular or sugary). Glass and plastic will have a much
greater translucency, with bubbles or flow lines often visible just below the
surface. Staining between grain boundaries may be visible in dyed imitations.
Some destructive tests may, however, be
necessary; for example, the application of diluted hydrochloric acid will cause
the carbonates odontolite and magnesite to effervesce and howlite to turn
green, while a heated probe may give rise to the pungent smell so indicative of
plastic. Differences in specific gravity, refractive index, light absorption
(as evident in a material's absorption spectrum), and other physical and
optical properties are also considered as means of separation.
Treatments
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. Gemologists can detect these treatments using a variety
of testing methods, some of which are destructive, such as the use of a heated
probe applied to an inconspicuous spot, which will reveal oil, wax or plastic
treatment.
Waxing and oiling
Historically, light waxing and oiling
were the first treatments used in ancient times, providing a wetting effect,
thereby enhancing the colour and lustre. This treatment is more or less
acceptable by tradition, especially because treated turquoise is usually of a
higher grade to begin with. Oiled and waxed stones are prone to
"sweating" under even gentle heat or if exposed to too much sun, and
they may develop a white surface film or bloom over time. (With some skill, oil
and wax treatments can be restored.)
Stabilization
Material treated with plastic or water
glass is termed "bonded" or "stabilized" turquoise. This
process consists of pressure impregnation of otherwise unsaleable chalky
American material by epoxy and plastics (such as polystyrene) and water glass
(sodium silicate) to produce a wetting effect and improve durability. Plastic
and water glass treatments are far more permanent and stable than waxing and
oiling, and can be applied to material too chemically or physically unstable
for oil or wax to provide sufficient improvement. Conversely, stabilization and
bonding are rejected by some as too radical an alteration.[16]
The epoxy binding technique was first
developed in the 1950s and has been attributed to Colbaugh Processing of
Arizona, a company that still operates today. The majority of American material
is now treated in this manner although it is a costly process requiring many
months to complete.[citation needed]
Dyeing
The use of Prussian blue and other dyes
(often in conjunction with bonding treatments) to "enhance” its
appearance, make uniform or completely change the colour, is regarded as
fraudulent by some purists,[16] especially since some dyes may fade or rub off
on the wearer. Dyes have also been used to darken the veins of turquoise.
Reconstitution
Perhaps the most extreme of treatments
is "reconstitution", wherein fragments of fine turquoise material,
too small to be used individually, are powdered and then bonded with resin to
form a solid mass. Very often the material sold as "reconstituted"
turquoise is artificial, with little or no natural stone, made entirely from
resins and dyes. In the trade "reconstituted" turquoise is often
called "block" turquoise or simply "block."
Backing
Since finer turquoise is often found as
thin seams, it may be glued to a base of stronger foreign material for
reinforcement. These stones are termed "backed," and it is standard
practice that all thinly cut turquoise in the Southwestern United States is
backed. Native indigenous peoples of this region, because of their considerable
use and wearing of turquoise, have found that backing increases the durability
of thinly cut slabs and cabochons of turquoise. They observe that if the stone
is not backed it will often crack. Early backing materials included the casings
of old model T batteries, old phonograph records, and more recently epoxy steel
resins. Backing of turquoise is not widely known outside of the Native American
and Southwestern United States jewellery trade. Backing does not diminish the
value of high quality turquoise, and indeed the process is expected for most
thinly cut American commercial gemstones.[citation needed]
Valuation and care
Slab of turquoise in matrix showing a
large variety of different colouration
Hardness and richness of colour are two
of the major factors in determining the value of turquoise; while colour is a
matter of individual taste, generally speaking, the most desirable is a strong
sky to robin egg blue (in reference to the eggs of the American robin).[9]
Whatever the colour, for many applications, turquoise should not be soft or
chalky; even if treated, such lesser material (to which most turquoise belongs)
is liable to fade or discolour over time and will not hold up to normal use in
jewellery.
The mother rock or matrix in which
turquoise is found can often be seen as splotches or a network of brown or
black veins running through the stone in a netted pattern; this veining may add
value to the stone if the result is complementary, but such a result is
uncommon. Such material is sometimes described as "spiderweb matrix";
it is most valued in the Southwest United States and Far East, but is not
highly appreciated in the Near East where unblemished and vein-free material is
ideal (regardless of how complementary the veining may be). Uniformity of
colour is desired, and in finished pieces the quality of workmanship is also a
factor; this includes the quality of the polish and the symmetry of the stone.
Calibrated stones—that is, stones adhering to standard jewellery setting measurements—may
also be more sought after. Like coral and other opaque gems, turquoise is
commonly sold at a price according to its physical size in millimetres rather
than weight.
Turquoise is treated in many different
ways, some more permanent and radical than others. Controversy exists as to
whether some of these treatments should be acceptable, but one can be more or
less forgiven universally: This is the light waxing or oiling applied to most
gem turquoise to improve its colour and lustre; if the material is of high
quality to begin with, very little of the wax or oil is absorbed and the
turquoise therefore does not "rely" on this impermanent treatment for
its beauty. All other factors being equal, untreated turquoise will always
command a higher price. Bonded and "reconstituted" material is worth
considerably less.
Being a phosphate mineral, turquoise is
inherently fragile and sensitive to solvents; perfume and other cosmetics will
attack the finish and may alter the colour of turquoise gems, as will skin
oils, as will most commercial jewellery cleaning fluids. Prolonged exposure to
direct sunlight may also discolour or dehydrate turquoise. Care should
therefore be taken when wearing such jewels: cosmetics, including sunscreen and
hair spray, should be applied before putting on turquoise jewellery, and they
should not be worn to a beach or other sun-bathed environment. After use,
turquoise should be gently cleaned with a soft cloth to avoid a buildup of
residue, and should be stored in its own container to avoid scratching by
harder gems. Turquoise can also be adversely affected if stored in an airtight
container.
See also
icon Gemology
and Jewelry portal
List of minerals
Bisbee Blue