What is Turquoise?
Turquoise is an opaque mineral that occurs in beautiful shades of blue, bluish green, green, and yellowish green. It has been treasured as a gemstone for thousands of years. Isolated from one another, the ancient people of Africa, Asia, South America and North America independently made turquoise one of their preferred materials for producing gemstones, inlay, and small sculptures.
Chemically, turquoise is a hydrous phosphate of copper and aluminum (CuAl6 (PO4)4 (OH)8·4H2O). Its only important use is in the manufacture of jewelry and ornamental objects. However, in that use it is extremely popular - so popular that the English language uses the word "turquoise" as the name of a slightly greenish blue color that is typical for high-quality turquoise.
Very few minerals have a color that is so well known, so characteristic, and so impressive that the name of the mineral becomes so commonly used. Only three other minerals - gold, silver, and copper - have a color that is used more often in common language than turquoise.
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Blue minerals are rare, and that is why Turquoise captures attention in the gemstone market. The most desirable color of Turquoise is a sky blue or robin's-egg blue. Some people inappropriately describe the color as "Persian blue" after the famous high-quality material mined in the area that is now known as Iraq. Using a geographic name with a gem material should only be done when the material was mined in that locality. The crystals are microscopically small and can hardly ever be recognised with the naked eye.
After blue, bluish green stones are preferred, with green and yellowish green material being less desirable. Departure from a nice blue color is caused by small amounts of iron substituting for aluminum in the Turquoise structure. The iron imparts a green tint to the Turquoise in proportion to its abundance. The color of Turquoise might also be altered by small amounts of iron or zinc substituting for copper in the Turquoise structure.
Some Turquoise contains inclusions of its host rock (known as matrix) that appear as black or brown spider-webbing or patches within the material. Many cutters try to produce stones that exclude the matrix, but sometimes it is so uniformly or finely distributed through the stone that it cannot be avoided. Some people who purchase Turquoise jewelry enjoy seeing the matrix within the stone, but as a general rule, Turquoise with heavy matrix is less desirable.
Some Turquoise localities produce material with a characteristic color and appearance. For example, the Sleeping Beauty Mine is known for its light blue Turquoise without matrix. Much of the Turquoise from the Kingman Mine is bright blue with a spider web of black matrix. The Morenci Mine produces a lot of dark blue Turquoise with pyrite in the matrix. Much of the Bisbee Turquoise has a bright blue color with a chocolate brown matrix. People who know Turquoise can often, but not always, correctly associate a stone with a specific mine.
Turquoise Jewelry Care
Turquoise has a lower-than-ideal hardness and durability for use in certain types of jewelry. Although the gem is frequently used in rings, bracelets, and belt buckles, these uses place the gem at risk for abrasion and impact. Because of their sensitivity, turquoises are almost always subjected to treatment of one kind or another, though this may take any of a number of different forms.
Turquoise is often porous, with the ability to absorb liquids. These liquids can include perspiration, body oil, cleaning products, or any liquid that accidentally comes in contact. Once absorbed, these liquids can damage the turquoise or alter its color. If contact with cosmetics or potentially damaging liquid occurs, the turquoise should be washed with a soft cloth that is dampened with a very mild soap solution, followed by cleaning with a soft cloth that has been dampened with plain water. Then, after the turquoise is dry, store it in a jewelry box away from bright light or heat.
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Turquoise is rarely found in well-formed crystals. Instead it is usually an aggregate of microcrystals. When the microcrystals are packed closely together, the Turquoise has a lower porosity, greater durability, and polishes to a higher luster. This luster falls short of being "vitreous" or "glassy." Instead many people describe it as "waxy" or "subvitreous."
Turquoise forms best in an arid climate, and that determines the geography of Turquoise sources. Most of the world's Turquoise rough is currently produced in the southwestern United States, China, Chile, Egypt, Iran, and Mexico. Turquoise occurs as a fillung in veins or crevices, or in the form of nuggets. The most well known deposits are in the USA, Mexico, Israel, Iran, Afghanistan and China. The most beautiful turquoises, in a splendid light blue, come from deposits in the north of Iran.
In these areas, rainfall infiltrates downward through soil and rock, dissolving small amounts of copper. When this water is later evaporated, the copper combines with aluminum and phosphorus to deposit tiny amounts of Turquoise on the walls of subsurface fractures.
Turquoise can also replace the rock in contact with these waters. If the replacement is complete, a solid mass of Turquoise will be formed. When the replacement is less complete, the host rock will appear as a "matrix" within the turquoise. The matrix can form a "spider web," "patchy" design, or other pattern within the stone.
Physical Properties of Turquoise
The physical properties of Turquoise are valuable for its identification. They are also valuable information for the care of Turquoise jewelry. Important properties are summarized in the accompanying table.
Weathering can significantly alter the physical properties of turquoise. Weathered Turquoise might still have a desirable color, but its hardness and durability are reduced. This Turquoise cannot be cut into useful cabochons or beads. Weathered Turquoise is often crushed and used to make "composite" or "reconstituted" Turquoise described in the "Natural Turquoise and Turquoise Treatments" section below.
The Turquoise Group of Minerals
The Turquoise group consists of five triclinic minerals. These minerals are very similar in chemical composition, crystal structure, physical properties and often in appearance. Members of the group are: turquoise, aheylite, chalcosiderite, faustite, and planerite. Their compositions are listed in the accompanying table.
Notice that the members of the Turquoise group have very similar chemical compositions. In these minerals iron often substitutes for aluminum, and copper often substitutes for zinc or iron. Because they are so similar and have ranges of composition, these minerals are often misidentified. As a result, some material sold as Turquoise is actually another mineral member of the Turquoise group.
Howlite and magnesite are light gray to white minerals that often have markings that resemble the spider webbing seen in some turquoise. They can be dyed a turquoise blue color that makes them look very similar to natural turquoise. These dyed stones fooled many people when they first entered the marketplace and still are mistaken for genuine turquoise by unfamiliar buyers.
Dyed stones have damaged the market for genuine turquoise. They have been purchased with the thought that they were turquoise by many people and have produced uncertainty in the mind of many jewelry buyers. This causes some people to avoid turquoise jewelry.
Today dyed howlite and magnesite are still used to make mass-produced beads, cabochons, tumbled stones, and other turquoise look-alike items. They are almost ubiquitous in the marketplace. Be cautious if you see turquoise with a wonderfully blue and very uniform color.
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