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 (Pogue, 19__).
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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Related Indian Jewelry Articles by Lee and Eric Anderson
• “The
History of American Indian Jewelry”
• “The
Squash Blossom Necklace”
• “Turquoise
in Indian Jewelry”
• “How
the Quality of Turquoise Affects Its Use in Jewelry”
• “Stones
Used In Indian Jewelry”
• “The
History of Turquoise”
• “The
Origin and Occurrence of Turquoise”
• “Natural,
Stabilized, Treated, Fake, and Synthetic Turquoise”
• “Turquoise
Quality”
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