The Origin and Occurrence of Turquoise

An article by Lee Anderson

Turquoise consists of the chemical elements copper (Cu), aluminum (Al), phosphorus (PO4), and water (H2O). It is described as a “hydrous basic aluminum phosphate of copper” or a “hydrous aluminum phosphate colored by copper salts.” Its chemical formula is:

CuAl6 (PO44)8 4H2O,

although this varies widely. This molecular structure permits the inclusion of other elements, principally iron (Fe), calcium (Ca), magnesium (Mg), manganese (Mn), silicon (Si), and zinc (Zn). These additional elements, when incorporated in the molecular structure of turquoise, influence its color and hardness.

Turquoise is formed when the proper minerals, present in the proper proportions, are subjected to certain physical and chemical processes. These minerals are broken down, or “weathered,” from nearby “source” rocks and then dissolved, transported, and deposited in cracks, openings, and hollows in “host” rocks that lie beneath the surface. This mineral “solution” must remain in these host rocks for millions of years, at just the right pressures and temperatures, to form turquoise. (Keep in mind that over these enormous periods of time, mountains rise and wear away, and seas advance and recede.) It’s remarkable that a specific grouping of minerals could be subjected to the forces of pressure and temperature for such long periods, eventually forming something as beautiful as turquoise!

Turquoise usually forms in areas with some volcanic or thermal history. Most is found in volcanic rocks such as phyolite and trachyte; a lesser, but still significant amount, is found in intrusive (granite-like) rocks. Metamorphic and sedimentary rocks, on the other hand, are least likely to contain turquoise, although the turquoise found in the Sinai and in Australia occurs in sandstone and shale.

Most turquoise is found in “alteration zones” — areas where the native rocks have been altered by heat from magma or other thermal influences. This “hydrothermal” alteration is created when magma solutions from deep within the earth flow to the surface through fractures or pores, eventually changing the original rocks because of the intense heat and chemical exchange with the new rock (magma). This activity, coupled with the long weathering of the surface rocks through wind and water and their resulting chemical breakdown, creates the environment necessary for turquoise to form.

Another key geological process is “silicification.” It, too, is involves hydrothermal and intrusive alteration. Silica, a common associate of turquoise, is introduced into the turquoise deposit. This process, in addition to periods of intense heat, is responsible for the hardness of the turquoise and frequently the matrix as well.

In order for turquoise to form, several conditions must be met. First, there must be a source of copper, a relatively rare element. Collocated with this copper must be a source of phosphorus — usually the mineral apatite, which in turn is restricted to certain rocks (which are not all associated with copper). Phosphorus is typically leached from the apatite in the form of phosphoric acid. There must also be feldspar for the aluminum and deep hydrothermal alteration to break down these feldspars and free this aluminum. The copper is usually introduced into the “host” rocks by the rising hot magma. It readily oxidizes near the surface and, when in solution, reacts freely with the aluminum and phosphoric acid to form turquoise. At this time, other minerals may enter into the turquoise structure, creating color variations.

Other factors affect the creation of turquoise. For example, the best, hardest turquoise is usually found within 100 feet of the earth’s surface. Why? Well, turquoise sitting in a pocket waiting for someone to mine is subject to the elements. If it’s near the surface, it “dries out.” As it “dries,” it hardens; deeper formations are generally softer. Shallow deposits have less contact with the acids created by water percolating through the earth, so they are less likely to “soften” or become more porous. Some cases appear to contradict this observation, such as the Lone Mountain Mine in Nevada. In this area, tunneling along the vein has been very productive even below 100 feet. That’s because the rocks have been faulted to the side —the turquoise actually formed near the surface before faulting.

Undoubtedly, many similar formations have been lost forever as a result of the earth’s convolutions, which have sent the deposits deep into the crust.

References / Recommended Readings

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. )

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.

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

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

Posted in Turquoise Facts & History | Comments Off on The Origin and Occurrence of Turquoise

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.

Posted in Turquoise Facts & History | Comments Off on The Physical Properties of Turquoise

Natural, Stabilized, Treated, Fake, and Synthetic Turquoise

An article by Lee Anderson

This article discusses various types of turquoise as they relate to the jewelry industry.

Natural

This turquoise comes directly from the mine. It is cut, shaped, polished, and set into jewelry. Perhaps it had only been drilled, polished, and suspended on a necklace. In any event, the stone has had no man-made treatment or additives other than the polishing compounds to set off its luster. Most stones used in their natural state are very good to gem quality — in other words, hard and dense, with an inherent luster that does not lessen as it is exposed to its natural setting.

Stabilized

This is a natural turquoise, usually in nugget form, that is too porous or soft to hold a luster. It is therefore submerged into a stabilizing compound, frequently an epoxy resin. The natural capillary action of the porous stone draws this stabilizing compound throughout the stone. It is then dried. When thoroughly dried, it can be cut, drilled, cabbed, etc., and prepared for jewelry. Please note that the turquoise has not been altered; rather, the pores of the stone have been filled with a clear resin that makes the stone usable. If this type of turquoise were not on the market, many, many, jewelry artisans would be unemployed. It allows wide diversity. For example, necklaces of tiny turquoise beads now can be made and tiny inlay is possible. Colors will not change because the pores are sealed. It is not practical to use a high-grade natural stone for heishe; too much turquoise is wasted in the grinding, and the resultant bead will be fragile and eventually change color.

On the other hand, some stabilizing compounds can have color added. This causes the turquoise to assume a color that is not naturally inherent to that stone. This is referred to as “color shot” or “color stabilized,” a misleading term that implies the natural color is “stabilized.” This, of course, is not true; color has been added. This practice is not necessarily bad. Jewelry-making is an art, and this color enhancement can improve the appearance of the piece. It goes without saying that the value is less than that of naturally colored turquoise.

Treated

This form of color enhancement has existed for thousands of years. Pogue discusses writings on this subject that pre-date Christ. A common treatment is to submerge the stone in animal or vegetable oil and later air-dry it to give it a luster that did not previously exist. Unfortunately, this luster will not last long, and wearing the piece will likely leave oil stains. Many sellers have had to leave the area shortly after making such a sale! Even today, some turquoise merchants submerge the stone in water to enhance its color and weight.

Fake and Synthetic

People have been faking turquoise for centuries using ceramics, bone, color-enhanced minerals, and more recently, celluloid and plastic, among other things. This “fake” turquoise not much of a problem now, as people are simply too familiar with turquoise. However, “synthetic” turquoise, frequently chemically perfect, has appeared on the market in some quantity. This is literally stove-top turquoise. It has a very natural matrix created by placing stones in the “batter” or sprinkling in pyrite, etc. When the mix is cut, then cabbed, these foreign additives, which are real, add to the illusion that the entire stone is natural. Synthetics become fake if not properly identified

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.

Posted in Turquoise Facts & History | Comments Off on Natural, Stabilized, Treated, Fake, and Synthetic Turquoise

Turquoise Quality

An article by Lee Anderson

We could write pages on this and still not cover the subject. In the earliest of times — up to the late 1800s — certainly, the pure blue without matrix was considered the best. It was time-tested; if the color did not change it was “old rock”…in other words, a gem! If the color did change, it was “new rock,” inferior and impermanent. While the pure blue stone could be gem quality, the matrixed stone was not considered a gem stone. This all changed in the late 19th century, when the American Indian developed a preference for the matrixed stone, causing an entire new grading criteria to evolve.

Today, the preferred turquoise in the Middle East is still the pure blue. As such, great quantities of rather inferior stabilized “chalk” turquoise have been shipped there for sale to local jewelers and merchants. People have brought me pieces they bought in bazaars and markets as “old Persian or Arab” jewelry that was actually made from Kingman stabilized turquoise. Beware!

Criteria for Grading Turquoise in the U.S.

Hardness / Density. This is a critical factor in determining the grade of a turquoise specimen. An inferior, chalk-like turquoise will feel light; it will be porous and stick to your tongue. The harder, denser pieces will have a “good” substantive feel to them. They will not draw the same quantity of moisture from your tongue as lower grades, but you will feel some adhesion to your tongue. As density increases, so too does hardness. Just as turquoise varies from a little over 2 to nearly 6 on the Mohs scale, its specific gravity also varies but typically is 2.8, like quartz.

Luster. This should come from within the stone — not just from a surface polish.

Color. No area is less codified than this. The ancients preferred blue because a gem-grade blue stone would not change color (King Tut’s treasures include a substantial amount of blue turquoise — it appears today unchanged). Because the softer blue stones would eventually start turning greenish, it was assumed that green was not as good. Time has proven this wrong. Some green-hued turquoise such as Skyhorse, China Mountain (both are names given to turquoise from China), Cerillos, Blue Gem, and Fox, to name a few, are ranked in the top three grades, like blue stones from the Lander, Lone Mountain, Red Mountain, Morenci, and Bisbee mines. To make matters even more difficult, some mining areas — such as Skyhorse, China Mountain, Blue Gem, and Royston — produce both colors.

Matrix. This is the host rock in which the turquoise forms and bonds. When cut, the host rock and the turquoise are one piece. The pattern of this matrix must be pleasing. This is subjective at best, but with experience, you learn what most people consider most desirable. Again, as in color, the opinions on which matrix is “best” varies dramatically. There are hard-core supporters of the fine, dark, spider web found in the Lander, Number Eight, Lone Mountain, Red Mountain, Skyhorse, and China Mountain mines. The heavy brown-black matrix of Bisbee and Tyrone has followers who believe it is the world’s best. A hard, lustrous cabochon from Morenci typifies another beautiful and highly regarded matrix. It is freeform, with a blending of webbing and deep pattern matrix combined with visible pyrite inclusions.

In a given cabochon of turquoise, any of the above could qualify as “best.” depending on the personal preferences of the one judging. However, when mounted in jewelry, you must consider the balance of the turquoise in the setting by itself or in combination with other stones.

Rarity. People covet that which is rare, and value escalates accordingly. A stone from a mine that produced a highly collectible stone that has subsequently closed appeals more to a collector then a stone from an active mine. Again, the other factors discussed above must also apply; rarity is simply the price discriminator. For example, a beautiful five-carat cabochon of deep-blue turquoise with a tiny black spider web matrix from the Lander Mine in Nevada (closed many years ago) has a retail value of $2,000 to $2500. A similar cabochon from the Lone Mountain Mine, also closed, would be $750 to $800. It is every bit as good — and, in the case of the matrix, better — but the mine produced for a longer time. A similar cabochon from the Skyhorse Mine from Tibet and China would be nearer $75 because it is still active and produces large quantities of turquoise.

Grades of Turquoise

Turquoise grading, again, is a subjective area; however, the following criteria are accepted by a good many in the trade. Note that percentages cited below should not be taken as exact. They are a “best guess” based on our experience of over 35 years.

1. Gem. For a stone to be considered a gem, all of the criteria listed above must be met except the rarity factor. Less then 1 percent of all turquoise can be legitimately called “gem.” Remember — rarity affects value, not natural quality.

2. Very High Grade. Stones of this grade are nearly perfect and exhibit the same general characteristics as gems, except that the matrix patterns may not be perfectly balanced. The stone would still be quite hard and lustrous. About 3 percent of all turquoise is very high grade.

3. High Grade. Turquoise of this grade is used in most high- but not competition-quality jewelry. It is hard but not as hard, balanced but not perfect — in other words, a very attractive specimen that could be just a bit better. Luster must be perfect. About 5 percent of turquoise fits this grade.

4 a/b/c. Jewelry Quality / High Quality / Investment Quality (note that the word “grades” is omitted). It should have a good hardness and feel, and it should not need stabilization. It must have a nice luster but not necessarily be as deep as higher grades. The matrix pattern should be attractive but probably a bit unbalanced. Although this stone could be stabilized to prevent color change, doing so is unnecessary because it will change slowly yet remain attractive nonetheless. Approximately 10 percent of turquoise is in this category.

5 a/b/c. Mine Run / Average Quality / Good Quality / Stock. This is a very average turquoise that needs no stabilization because it holds polish and stays attractive. Stabilizing, however, improves the stones by strengthening it for carving and permanence. We estimate that about 20 percent of turquoise falls within this category.

6 a/b/c. Chalk / Bulk / Chip Stock / “Levarite” (as in “leave ‘er right there”). This stone is soft, porous, brittle, and of little value to the jewelry industry until stabilized. Frequently, the color is insufficient, so pieces are “color enhanced” or “color shot” — in other words, artificially colored. Most turquoise falls into this category. One reason is that mines have needed to become deeper as shallow turquoise deposits have been removed. As our article on the origin and occurrence of turquoise indicates, the deeper the deposit, the lower the quality.

Conclusion

Turquoise is considered a precious stone. At one time in history, superior specimens were valued by weight, more than gold. Today, turquoise ranges from a few cents per carat (chalk) to over $600 per carat for a superb gem stone. It is widely regarded as our nation’s “national stone.” Man has coveted, romanced, fought for, and owned this remarkable stone with pride.

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.

Posted in Turquoise Facts & History | Comments Off on Turquoise Quality