The Physical Properties of Turquoise

An article by Lee Anderson 

Hardness

Turquoise is opaque and has a Mohs scale hardness that varies remarkably. The soft, deeply mined chalks may only slightly exceed 2 on the Mohs scale, whereas a hard, gem specimen may exceed 6. The hardness varies in response to several factors, including environment and matrix. The silicification process, which can produce gem stones, usually involves very minute quartz particles. This process will strengthen some of the matrices as well. If silicification has not occurred, the turquoise will likely be chalky, porous, and soft. It will not be usable in jewelry without undergoing treatment — usually stabilization.

Specific Gravity

The specific gravity of turquoise is 2.6 to 2.8, about the same as quartz. Of course, when some minerals replace other minerals, the specific gravity will change/

Color

The color of turquoise can vary from deep blue to deep green, with every variation of color in between, because of its chemical composition. Generally, the more copper in the molecule, the bluer the turquoise. The introduction of iron causes a greener cast. Turquoise specimens from various mines have been analyzed to determine their chemical compositions. Generally, these analyses support this color generalization. Turquoise can also change color naturally, usually becoming greener when exposed to moisture. This can occur when the stone is in the ground or when it is used in jewelry. The process is similar to what happens to blue azurite, which changes to green malachite when water content increases in its creation environment. Man, too, can change the color of turquoise artificially by submerging the stone in animal fat. This has been done for centuries simply to make it prettier and to increase its value in trade. Wetting the stone in water immediately prior to sale makes the color more pronounced and the stone heavier; both techniques, however, are temporary. We discuss prominent color change later in this article.

Although turquoise must consist of copper, aluminum, and phosphorus, other elements can replace them (in various percentages), thereby changing the molecular structure. For example, two very rare minerals, chalcosiderite (where iron replaces the aluminum) and faustite (where zinc replaces the aluminum) exist in turquoise environments. However, more frequently, iron and zinc will partially replace the aluminum, leaving turquoise altered only in color, specific gravity, and of course, chemical composition. Most turquoise is concentrated more near the copper-aluminum end of this spectrum than the iron or zinc-aluminum end; therefore, most turquoise is blue or blue-green. A great many variables exist in this stone; it is sill turquoise. No one factor makes it more or less valuable.

A series of chemical tests were conducted on turquoise from 21 different mines in several countries. There were marked differences in the composition of the oxides forming these turquoise samples. For example, copper ranged from 1.4 percent in a Persian mine to a 9 percent in Virginia; phosphorus was 14 percent in the Persian samples but 39 percent in Jordan; and aluminum varied from a low of 29 percent in New Mexico’s Cerillos mine to a high of 54 percent in Jordan. Averages for the 10 U.S. samples were as follows: copper, 4–9 percent; phosphorus, 27–34 percent; and aluminum, 29–44 percent.

The tests for iron ranged from zero percent in three mines to 7.8 percent in Persia. U.S. mines ranged from 1.2 to 4.4 percent. Water, a key element, averaged 18 percent in all mines .

These tests, if nothing else, show just how complex a mined turquoise is; turquoise simply does not always follow the accepted generalizations. For example, the tests show that a Persian mine noted for its blue turquoise had the lowest copper and highest iron content. This appears to contradict the generalization that bluer stones contain more copper. The tests revealed traces of other oxides that affect color. Lastly, these tests were conducted many years ago and we know today that ore samples from a single mine can vary rather markedly.

References / Recommended Readings

John Adair, The Navajo and Pueblo Silversmiths, University of Oklahoma Press, 1944.

Margery Bedinger, Indian Silver, Navajo and Pueblo Jewelers, University of New Mexico Press, 1973.

M.G. Brown, Blue Gold, The Turquoise Story, Main Street Press, Anaheim, CA, 1975.

Larry Frank, Indian Silver Jewelry of the Southwest, Schiffer Publishing Ltd., Westchester, Pennsylvania, 1990.

The International Turquoise Annuals, vol. I and II, 1975 and 1976 (only two published) Impart Pub, Reno, NV. Note in vol. I the article on pages 31–55 by D. Allen Penick, “Turquoise, the Mineral that’s an Accident.”

Carl Rosnek and Joseph Stacy, Skystone and Silver, Prentice Hall, Englewood Cliffs, New Jersey, 1976.

Joseph E. Poque, Ph.D, The Turquoise, A report to the National Academy of Science, vol. XII, Second and Third Memoir, 1915. Reprinted in 1974 by Rio Grande Press, Inc., Glorieta, NM. (This reprint includes a foreword and details on Southwestern turquoise mines by Rex Arrowsmith and an excellent reference list. )

Stuart A. Northrop, Turquoise and Spanish Mines in New Mexico, University of New Mexico, Press, 1975.

Stuart A. Northrop, David L. Newman, David H. Snow, Turquoise, reprinted by General Printing and Paper Co., Topeka, KS. A reprint from El Palacio, vol. 79, No. 1, 1973, Museum of New Mexico.

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