Properties of a gemstone mineral that has been held in high regard for thousands of years.
What is Turquoise?
Turquoise is an opaque mineral that occurs in beautiful hues of blue, blue-green, and yellow-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)8·5H2). 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 blue-green color that matches the stone. Very few minerals have a color that is so well known, so characteristic and 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 in common language more than turquoise.
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.
After blue, blue-green stones are preferred, with 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. Turquoise, especially the more porous varieties, can discolor with exposure to prolonged sunlight, heat, cosmetics, perspiration, and body oil.
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 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." Porous turquoise is sometimes treated by soaking it in melted wax or impregnating it with polymer plastic to improve its characteristics.
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.
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
|Color||Sky blue (most desirable as a gemstone), blue, blue-green, yellowish green, often with brown, black or metallic matrix spider-webbing through the material|
|Luster||Waxy to subvitreous|
|Mohs Hardness||5 to 6|
|Specific Gravity||2.6 to 2.9 (variable because of porosity)|
|Uses||Decorative stone, gemstone|
The Turquoise Group of Minerals
The turquoise group consists of five monoclinic minerals with a similar chemical composition and structure. Included are turquoise, aheylite, chalcosiderite, faustite, and planerite. Their compositions are listed below.
Turquoise Group Minerals
Turquoise in the United States
Most of the turquoise production in the United States has been located in the arid southwest, and most of that production has been in or around deposits of copper. Arizona, New Mexico, and Nevada have all held the position of the leading turquoise-producing state. New Mexico held that position until the 1920s, Nevada held the position until the 1980s, and Arizona is currently the leading state. Significant amounts of turquoise have been produced in California, Colorado, Utah, Texas, and Arkansas.
Most of the turquoise is a byproduct of copper production. The large open-pit copper mines excavate down through the shallow rock units where the turquoise is formed. When turquoise is encountered, the quantity and quality of the material is assessed, and if warranted, a temporary effort is made to recover the gem material. If the value of the turquoise is worth disrupting the mining operation, it will be mined. The mining could be done by copper company employees, but the job is often given to contract miners who are able to come in and quickly recover the turquoise.
Turquoise Jewelry and Art
The earliest record of turquoise being used in jewelry or in ornaments is from Egypt. There, turquoise has been found in royal burials over 6000 years old. About 4000 years ago, miners in Persia produced a blue variety of turquoise with a "sky blue" or "robin's-egg blue" color. This material was very popular and traded through Asia and into Europe. This is the source of the term "Persian Blue" color.
In North America the earliest known use of turquoise was in the Chaco Canyon area of New Mexico, where the gem was used over 2000 years ago. Ancient artists produced beads, pendants, inlay work, and small sculptures.
Rough turquoise and turquoise objects were held in high regard by Native Americans and were traded widely. This spread North American turquoise across the southwest and into South America. These early Native American jewelry designs were simple, and the turquoise was not set in metal findings.
In the late 1800s, Native American artists began using coin silver to make jewelry. This work evolved into the turquoise and sterling silver style of Native American jewelry that is popular today.
The demand for turquoise and turquoise jewelry rises and falls over time. In the United States there was a surge in demand that began in the 1970s and declined in the 1980s. Demand for turquoise jewelry is always highest in the southwestern states where turquoise mining and Native American artists make turquoise part of the local culture.
Synthetic and Imitation Turquoise
A small amount of synthetic turquoise was produced by the Gilson Company in the 1980s, and some of their material was used to make jewelry. It was produced in a sky blue color, sometimes with a gray spider webbing. It was a ceramic product with a composition similar to natural turquoise.
Synthetic turquoise and turquoise simulants have been produced in Russia and China since the 1970s. They are prolific producers. The material is used to make cabochons, beads, small sculptures, and many other items. A photo on this page shows some synthetic turquoise cabochons made in Russia.
There are many different glass, plastic, and ceramic materials with an appearance similar to turquoise. Many of these can easily be distinguished from turquoise by testing their hardness, specific gravity, or other properties.
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. The dye generally does not penetrate deeply into the material. Scratching the back of a stone with a pin will often reveal a white interior. When heavily dyed, a stone must be scratched deeply or be broken to reveal the light interior.
A few minerals are sometimes confused with turquoise by people who are unfamiliar with turquoise. These minerals include: variscite, larimar, blue-green chalcedony, lapis lazuli, and chrysocolla. Simple tests will usually differentiate these materials; however, some of the tests are destructive and are usually not suitable for use on finished jewelry items.
Contributor: Hobart King
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