By Robert Weldon

Dayton Simmons brought a personal collection of natural and treated turquoise so staff and students could feel the differences between them. Photo by Robert Weldon/GIA

Simmons' presentation, "Turquoise: Natural or Otherwise," briefly outlined the uses of turquoise through time, such as in ancient Egypt, China and Persia, but with an emphasis on turquoise production and use in the Americas.

"Turquoise in the United States is often associated with the Navajo tribe," he noted, "but turquoise was used even earlier by the ancient Pueblo [also known as Anasazi] in Chaco Canyon in Northwestern New Mexico. It was also extensively traded throughout the Americas: Montezuma [the Aztec ruler] is said to have been buried in turquoise in Tenochtitlan, which is present-day Mexico City."

Simmons said that a 1980s Wall Street Journal article that referred to turquoise as "blue gold," caused a modern renaissance in turquoise appreciation – and prices escalated dramatically.

Simmons, who owns Silver Trading Company based in Santa Fe, New Mexico, also spoke briefly about the geologic conditions needed for turquoise to form.

"An aggregate of copper aluminum phosphate hydrate, turquoise forms under heat and pressure," he explained. "The various colors of turquoise can be attributed to the content of either copper or iron as coloring agents; a higher copper content yields bluer turquoise, whereas more iron causes greener colors."

He also encouraged audience members to learn to feel the difference between natural and treated turquoise. Natural turquoise tends to be more porous and not as shiny or slick as stabilized material, he said.

He allowed audience members to handle natural and treated material from his personal collection and noted that purchasing turquoise has become a gamble since the development of advanced turquoise treatments in the 1950s.

"The vast majority of turquoise available for purchase is treated, and is often sold without disclosure. The future of turquoise requires the buying public to be much more educated about their choices," he said. Rigorous testing to determine whether turquoise is natural, treated or reconstituted can be destructive: "Heat point tests may reveal plastic impregnations and damage natural turquoise," he explained, and natural turquoise may explode on contact with extreme heat.

He noted that visual examination can also be revealing. The matrix in veined turquoise is hardened during the stabilization treatment, and thus quite different from an untreated matrix; this feature can be identified by experts.

"For buyers who are not turquoise experts, the best advice is to know the dealer and to be comfortable with their methods of doing business," he said.

Simmons highlighted types of natural turquoise and common treatments:

• Natural turquoise – Untreated turquoise tends to be more porous than treated material and can often be brittle. Natural turquoise is often "backed" with a rigid layer to ensure durability.
• Stabilized turquoise – Natural turquoise impregnated with a binding agent that increases durability and darkens and saturates the color.
• Reconstituted turquoise – Fine particles of turquoise are mixed with acrylic and dye to form "blocks" that can be sawn, formed and polished. It often takes a very even, smooth polish.
• "Elgen" treated turquoise – A secret form of treatment (fewer than two years on the market) that employs a proprietary recipe to bind and stabilize turquoise. The treatment brings turquoise to a hardness of 7 on the Mohs Hardness Scale (natural turquoise is 5 to 6).