Turquoise is an opaque blue to green mineral made of copper, aluminum and phosphate with water in its structure. It is one of the oldest gem materials in use, mined in the Sinai and Iran thousands of years ago and later prized across the American Southwest. Most turquoise on the market is stabilized to harden it. Turquoise is one of the December birthstones.
What turquoise is
Turquoise has the formula CuAl6(PO4)4(OH)8·4H2O. Copper gives the blue. Iron replacing some aluminum shifts the color toward green, and some turquoise contains zinc as well. The finest color is a medium sky blue, often called robin's egg blue or Persian blue. Greener and paler stones are more common.
Individual turquoise crystals are tiny and rare. Gem turquoise is a microcrystalline aggregate, a mass of crystals too small to see. Its porosity depends on how tightly the crystals pack. Dense, hard turquoise takes a high polish and holds its color. Soft, chalky turquoise is porous, pale and absorbs oils and dirt.
How and where turquoise forms
Turquoise forms near the surface in dry climates. Water seeps through rocks containing copper, aluminum and phosphate minerals, usually near copper deposits. As the water evaporates, turquoise precipitates in cracks, veins and nodules. The sedimentary rocks and gemstones page covers this kind of near-surface formation. Malachite, azurite and chrysocolla often form in the same weathered zone.
Turquoise often grows in a host rock that becomes part of the gem. The host rock forms veins called matrix, which can be brown, black, golden or red. Spiderweb turquoise has thin matrix lines that form a net across the stone. Some buyers prefer clean, matrix-free stones, and others prize a fine spiderweb.
Persian turquoise
Iran's Nishapur district in the northeast has produced turquoise for at least two thousand years. Its best material is an even, intense sky blue with little or no matrix. For centuries this was the standard against which all turquoise was judged, and the term Persian blue still describes that color, wherever the stone comes from.
Persian turquoise decorated palaces, mosques, weapons, horse trappings and jewelry. The Persian jewelry page describes how it was used across Persian history. The word turquoise itself comes from Old French for Turkish stone, because the gem reached Europe through Turkish trade routes.
Egyptian and Chinese turquoise
Ancient Egyptians mined turquoise in the Sinai Peninsula, at sites such as Serabit el-Khadim and Wadi Maghara, from about the third millennium BCE. They set it in jewelry, collars and burial goods. The ancient Egyptian jewelry page covers its role.
China has mined turquoise for thousands of years, especially in Hubei province. By the late 20th century China had become a large supplier to world markets, including material that was sold in the American Southwest.
American Southwest turquoise
Native American peoples of the Southwest mined turquoise for centuries before European contact. The Cerrillos Hills near Santa Fe, New Mexico, were a major ancient source. Ancestral Puebloans traded turquoise widely, and the Chaco Canyon sites held large amounts of it. Navajo, Zuni, Hopi and Pueblo artists later combined turquoise with silver, a tradition that grew from the late 19th century. The traditional and tribal jewelry page covers Navajo and Zuni work.
Many American mines are known by name, and the mine name helps describe and value a stone:
- Sleeping Beauty, near Globe, Arizona, produced an even, sky-blue turquoise with little or no matrix. The mine stopped turquoise production in 2012.
- Kingman, Arizona, produced bright blue turquoise, often with black matrix, and has been one of the largest American sources.
- Bisbee, Arizona, a copper mine, produced deep blue turquoise with chocolate brown to red matrix. It is now scarce.
- Nevada has had many mines, including Number Eight, Lander Blue and Royston, known for spiderweb, green and mixed colors.
Stabilization and other treatments
Most turquoise is soft and porous, so most of it is treated. The gemstone treatments page covers these methods in more detail.
- Stabilization fills pores with a clear resin under pressure. It hardens the stone, deepens the color and lets soft turquoise take a polish. Stabilized turquoise is widely accepted when disclosed.
- Waxing and oiling are old, surface-level treatments that darken the color temporarily.
- Dyeing adds or deepens color, often in porous stones.
- The Zachery treatment, a proprietary method developed in the United States in the 1980s, improves color and hardness and is hard to detect.
Natural, untreated turquoise is rarer and costs more. Labs can detect most resins and dyes with infrared spectroscopy.
Reconstituted turquoise and imitations
Reconstituted turquoise is made from turquoise fragments or powder bonded with resin and pressed into blocks. It is real turquoise in a sense, but not a natural stone, and must be disclosed.
Imitations are common:
- Dyed howlite and dyed magnesite are white, porous minerals dyed blue. Both are widely sold as turquoise. They often show color concentrated in cracks and look chalky inside.
- Plastic and glass imitations are lighter or show bubbles.
- Gilson produced a manufactured turquoise in the 1970s that gemologists can separate from natural stone under magnification.
- Block turquoise is a vague trade term often used for imitation or reconstituted material.
A gemologist identifies turquoise by refractive index, specific gravity, spectroscopy and magnification. Because imitations and treatments are so common, buy from sellers who clearly disclose treatments and identify the material.
Durability and care
Turquoise has a hardness of 5 to 6. It is porous and absorbs oils, cosmetics, perfume and soap, which can turn it green over time. Heat can dry it out and change its color. Turquoise is not suited to ultrasonic or steam cleaners.
Wipe turquoise with a soft, damp cloth and dry it. Put it on after applying lotion and perfume. Store it away from harder gems, and keep it out of strong heat and sunlight for long periods. The guide to cleaning jewelry safely covers porous gems.
Turquoise as a December birthstone
Turquoise is a traditional December birthstone, and it remains on the modern list along with zircon and tanzanite. The birthstones by month page has the full list.
Frequently asked questions
Why does turquoise turn green?
Porous turquoise absorbs skin oils, soap and cosmetics, and loses water when heated. Both can shift blue turquoise toward green over time.
Is stabilized turquoise real turquoise?
Yes. It is natural turquoise with resin added to fill pores. It should be sold as stabilized.
How can you tell dyed howlite from turquoise?
Dyed howlite often shows dye collected in cracks, white spots inside, and a chalky look. A gemologist can confirm it with refractive index and specific gravity.
What is the most prized turquoise?
Even, intense sky-blue turquoise with high hardness, such as fine Persian or Sleeping Beauty material, is widely prized. Some buyers prefer fine spiderweb stones from historic American mines.