Physical Properties Of Turquoise

Physical Properties of Turquoise

 

Chemical Classification Phosphate
Color Sky blue (the most desirable as a gemstone), blue, bluish green, green, yellowish green; often with brown or black matrix, spider webbing or background color.
Streak Bluish white to greenish white
Luster Waxy to subvitreous
Diaphaneity Opaque
Cleavage Perfect, but rarely seen because of the small grain size of most specimens
Mohs Hardness 5 to 6 (often lower because of porosity)
Specific Gravity 2.6 to 2.9 (variable because of porosity and matrix)
Diagnostic Properties Color, refractive index
Chemical Composition CuAl6(PO4)4(OH)8·4H2O
Crystal System Triclinic
Uses Gemstone, small sculptures, decorative stone

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 below.

Turquoise Group Minerals
Mineral Chemical Composition
Turquoise CuAl6(PO4)4(OH)8·4H2O
Aheylite (Fe,Zn)Al6(PO4)4(OH)8·4H2O
Chalcosiderite Cu(FeAl)6(PO4)4(OH)8·4H2O
Faustite (Zn,Cu)Al6(PO4)4(OH)8·4H2O
Planerite Al6(PO4)2(OH)8·4H2O

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.

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 mined in the United States 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, only if warranted, will a temporary effort be made to recover the gem material. If the value of the turquoise is worth disrupting a billion dollar mining operation, it will be mined. The mining could be done by copper company employees, but the job often goes to outside miners who come to the mine at a moment’s notice, quickly recover the turquoise, and get out of the way!

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’segg 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 1912, turquoise was named as one of the original modern birthstones for the month of December. This gave the gem a small boost in popularity which continues to the present.
In the United States there was a surge in turquoise 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 a distinctive part of the local culture.

Natural Turquoise and Turquoise Treatments
Only a small amount of turquoise that is mined today can be used to cut finished stones or make jewelry without some type of treatment. These treatments make the turquoise stable enough for cutting, durable enough for jewelry, or improve its color and marketability.
Untreated turquoise is a rare commodity. The rough, the stones cut from it, and jewelry made from it are special and held in highest regard by many people. Untreated turquoise is hard to find, and lots of people want it for a variety of reasons.
The different types of turquoise, based upon their treatments, are described below. They are listed from top to bottom in order of their desirability.
Natural Untreated Turquoise is the name for turquoise that is fashioned into cabochons, beads or other items and used to manufacture jewelry without any type of treatment. It is the most highly desirable type of turquoise, especially when it has an attractive color. You are most likely to find it for sale in stores that specialize in fine turquoise jewelry, those that sell high-quality one-of-a-kind items, or those that specialize in natural, untreated gems. Sellers of natural untreated turquoise frequently use the fact that no treatment has been done as a selling point.
Stabilized Turquoise is the name used for turquoise that has been impregnated with a polymer or other binding material to make it durable enough for cutting and use in jewelry. Straight from the mine this turquoise is too soft, too porous, too fractured or too fragile for manufacturing. Stabilized turquoise can be cut into beautiful beads and cabochons. It is the most common type of turquoise in today’s gem and jewelry market. It is widely accepted because the supply of natural turquoise is much smaller than the demand from people who want it. The fact that an item is made from stabilized turquoise should always be disclosed to the buyer prior to sale, and the price should be lower than untreated turquoise of similar quality.
Composite or Reconstituted Turquoise is made from small pieces of turquoise that are mixed with a polymer and cast into block-shaped pieces. Finely crushed turquoise and some non-turquoise materials are sometimes included. The blocks are then sawn into small pieces that are used to cut cabochons, beads and other items. This material is often called “block turquoise” because of this manufacturing method. Sellers who pride themselves on selling only natural gems frequently decline to sell reconstituted turquoise. Some gem identification labs decline to call this material “turquoise” and instead label it as a “manmade product.” At the same time, some vendors are glad to sell these materials and jewelry made from them because they often have an attractive appearance and a low price. Dyed Turquoise is exactly that. Turquoise is a porous material and easily accepts dye. The dye is used to modify the turquoise to a more marketable color. Dye can also be used to produce an outrageous color. Composite and reconstituted turquoise are the most commonly dyed materials. Dye can be used to color the turquoise or to color the polymer binding material. Sometimes black or brown dye is used to alter the color of matrix material to make it more obvious and uniform. Dyed turquoise is always worth less than untreated material of a similar color and quality.

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. Both countries 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, refractive index, or other properties.

Look-Alike Materials
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.

Ending Some of the Turquoise Confusion
Howlite and magnesite can easily be separated from turquoise using a refractometer. They have different refractive indexes. If you are willing to do a destructive test, the blue dye used to color howlite and magnesite generally does not penetrate very deep. If lightly dyed, scratching the back of a cabochon with a pin will often reveal a white interior. If the piece is heavily dyed, scratching deeper or breaking an edge might be required to determine if the piece has been dyed.
Several minerals are found where turquoise is expected, look similar to turquoise, are misidentified as turquoise, and often enter the gem and jewelry market labeled as turquoise. Variscite, chalcosiderite, and magnesite are examples. A yellow, white, brown or green color should be an immediate clue that these might not be turquoise. An easy-todo refractive index test will quickly separate all of these minerals from turquoise.
Turquoise has a refractive index of 1.61 to 1.65. All of these other minerals are different as shown in the table below.

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