Prized for its sky-blue color, turquoise has been used in amulets and jewelry since antiquity. The oldest known piece of jewelry is a turquoise and gold bracelet found in the tomb of Zer, an Egyptian queen who lived 7,500 years ago.

Turquoise is an anglicized version of turkois, which is French for “Turkey stone.” Originally mined in Persia (now Iran), turquoise arrived in Europe via Turkey, hence the name. Persian turquoise is noted for the purity of its blue color and experts regard it as the finest of all turquoise. Most turquoise today comes from China, Mexico and the United States; it’s no longer mined in Iran.

Depending upon its origin, turquoise may vary in color from pale blue to bright green. Although pure sky blue is regarded as best by many people, there is no ideal color. Each hue has its devotees.

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Turquoise is found close to the surface in arid climates. Phosphate and aluminum combine to form turquoise when surface waters seep into underlying rocks. The blue comes from small amounts of copper, while iron impurities yield a greenish color. Although sometimes found in veins, turquoise most often occurs as nuggets.

The rock in which turquoise is formed is called a matrix. Turquoise with parts of the rock still embedded in it is called matrix turquoise. Nuggets may also contain small bits (inclusions) of pyrite or other minerals. Iron oxide forms a webbed pattern of fine lines, leading to the name “spiderweb” turquoise. Veining or a mottled appearance in the stone is from the uneven distribution of copper.

Natural turquoise is turquoise which has had nothing done to it that changes or preserves the color, hardens or protects the surface or makes it easier to work with. It’s usually just polished and placed in a setting.

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Because it is porous, natural turquoise is absorbent. Perfumes, cosmetics, and perspiration can cause it to change color or become dull. Excessive heat will destroy turquoise. It’s also a very soft stone that requires care in wearing.

Turquoise is frequently treated in several ways, some of which are nearly undetectable, even by experts. Treating turquoise often preserves the color and protects the stone. It also salvages otherwise unusable stones. Artificial, imitation and synthetic turquoise are also on the market. As with treated turquoise, buying these is only a problem if you think you’re buying natural stones.

The most common treatments are stabilization and enhancement. Most stabilized turquoise has been impregnated with clear plastic. Stabilized turquoise is no longer porous or subject to color changes. It also allows holes to be drilled in nuggets or beads for stringing. If a dye is added to the plastic, then the turquoise is enhanced. Both stabilization and enhancement are permanent, unless wax was used rather than plastic.

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Thin pieces of turquoise may be backed with epoxy or artificial matrix to add thickness or strength. Done well, the backing is impossible to detect once the stone is set; however, it may dissolve if cleaned in certain solvents.

Other, less-desirable treatments include dipping the stone in oil to temporarily improve the color, using black shoe polish to create “matrix” or “spiderwebbing” or using epoxy to fill holes or create “pyrite” inclusions. Dyed howlite may be sold as “turquosite.” Gilson turquoise is a high-quality synthetic.

There are few nondestructive ways to test turquoise. Heat and chemicals will destroy or damage both natural and treated turquoise. Your best defense is to buy from reputable dealers and ask lots of questions.

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Ancient people wore turquoise to protect them from a multitude of misfortunes, including disease, poison and snakes. It was especially used to protect horseback riders from falls. Many believed that turquoise had the power to attract love, wealth and beauty to those who wore it.