The name “turquoise” has an uncertain origin but most likely derives from a phrase meaning “Turkish stone,” possibly a reference to the Persian turquoise trade route, which passed through Turkey.

History and romance. Egypt’s first dynasty used turquoise in earrings, necklaces, rings, bracelets, anklets, belts, and headdresses. It also was carved into scarabs worn by priests and nobility.

Turquoise played an important part in Native American life, dating back to about 700 b.c. Native Americans saw in turquoise the blending of two spirits, sea and sky, and used it to bless their warriors and hunters. They also used turquoise for arrowheads, believing it sharpened their aim.

Until recently, the world’s finest turquoise came from the Neyshabur (Nishapur) mines in Iran, which, according to 12th-century writings, were opened by Isaac, son of Abraham. Some are still mined and still known as the Isaac mines. Turquoise became a major currency for the ancient Persians.

Chinese turquoise dates to 1200 a.d. It was of much lesser quality than the Persian variety and was used mainly for carvings and decorative art. The British brought Chinese turquoise to England in the 1800s, but it didn’t become fashionable until the time of Queen Victoria (1837-1901). Victorian and art nouveau jewelry often was set with large Persian turquoises, emeralds, and garnets.

Turquoise has been associated with good health. The copper content of the gem is said to give it excellent curative powers.

Color variations. Turquoise color can range from green to blue, depending on chemistry and porosity. In its pure state, it’s a copper aluminum phosphate. As a rule of thumb, the more copper, the more blue the turquoise. Iron imparts a green hue. Material with high purity and high density typically shows a medium saturated blue, with no change in color over thousands of years.

U.S. deposits of fine-quality blue gems include Morenci and Bisbee in Arizona. High-quality green-hued turquoises include Chinese, especially from Skyhorse Mine and China Mountain, and Southwestern U.S. finds at Cerillos, N.M., and Fox, Nev.

Porous turquoise is subject to infiltration by impurities, which can change the color to a greenish-blue. Turquoise in jewelry often changes color after years of contact with skin oils, perfumes, soapy water, and even silver polish.

Qualities. Two varieties of turquoise are considered top-quality, but both have a pure “robin’s egg” blue color. The historical preference is for a pure, even, continuous blue. That variety is still the most popular in the Middle East and Europe. Other turquoise connoisseurs, mainly in the Western Hemisphere, prefer to see a web-like black veining spread evenly throughout the blue color.

Matrix is the host rock in which turquoise is formed. Persian turquoise often can be cut to exclude host material. U.S. deposits more commonly include host rock intertwined with turquoise, and it’s virtually impossible to fashion domestic turquoise without including some matrix. For U.S. and Chinese turquoise, evenly distributed matrix, called spiderwebbing, is the key to quality.

Fine-quality, black spider web matrix is indicative of Lone Mountain, Skyhorse, and China Mountain mines. Thick brown and black matrix is typical of Bisbee and is considered by many to be the finest.

Value. Value depends not only on color but also on luster, which is affected by hardness and density. The hardness of turquoise varies from 2 to almost 6 on the Mohs scale. Density varies from 2.6 to 2.9. Higher density imparts a heavier feel. A fine-quality gem accepts polishing, which results in bright, smooth, and shining luster. A poor-quality gem won’t polish well, revealing pits, polish lines, and a dull finish.

Enhancements. For centuries, immersing turquoise in animal fat or vegetable oil gave the gem a “wet” look, making it more salable. Today, there are three main turquoise enhancements:

Zachery enhancement. An electrical engineer named James E. Zachery developed a process that adds potassium to turquoise without introducing fillers such as epoxy, wax, or polymers. The enhancement strengthens the gem and sometimes adds color, darkening the stone slightly. The Zachery enhancement can be identified only by X-ray fluorescence.

Stabilization. Common U.S. turquoise often occurs as a nugget that’s too porous or soft to hold a polish. The stone is hardened by immersion in a liquid stabilizer, usually an epoxy resin or polystyrene, which fills the pores. When the epoxy hardens, the gem can be cut, polished, faceted, and drilled. The filler also protects the natural color by keeping out chemicals.

Color stabilization. Some enhancements add color to turquoise by mixing a dye in the stabilizer. These treatments are also known as “color-infused,” “color-treated,” “color-enhanced,” and “color-shot.” The value of such turquoise is far less than that of naturally colored material.

Pricing. Turquoise costs from pennies per carat to more than $15 per carat for a top-quality stone. Turquoise from the Sleeping Beauty Mine in Arizona is considered among the finest in the world, not only because of its natural, matrix-free, even-colored quality but also because it can be improved through stabilization using the Zachery method. Typical prices for small cabochons are estimated at $3 to $4 per carat.

Some cabochons from the now-closed Lander Mine in Nevada can fetch $300 to $500 per stone. They’re deep blue and have a tiny black spider-web matrix. Extra-fine-quality cabochons from the Skyhorse Mine in China can cost $50 per stone. Chinese turquoise of not-so-fine quality can sell for as little as 30 cents per carat. A newly discovered deposit in New Mexico called the Lost Mine of Enchantment is expected to produce fine-quality gems that could sell for $3 to $10 per carat.

Care and cleaning. Turquoise can be damaged through wear, handling, and exposure to chemicals, including perspiration, cosmetics, and perfumes. It’s best not to wear it against the skin. Overexposure to light and heat can change the color to a greenish hue. Turquoise rings should be removed before washing hands. To clean turquoise, wipe it with warm soapy water.

Bench settings and precautions. Keep heat and liquid away from turquoise. Whenever possible, remove the stone from its mounting before working on turquoise jewelry.

Recommended reading. For more information, see the following references:

Emmanuel Fritsch, Shane F. McClure, Mikhail Ostrooumov, Yves Andres, Thomas Moses, John Koivula, and Robert Kammerling, “The Identification of Zachery-Treated Turquoise,” Gems & Gemology, Spring 1999, pp. 4-16.

Tanner Wheat, Clara Lee Wheat, and Joe Ben Wheat, Ray Manley’s Portraits & Turquoise of Southwest Indians (Ray Manley Photography Inc., 1975).