Friday, February 22, 2013

Looking Beyond A Gem's Beauty

Scientists use multiple tests to prove the region where gems were mined.

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Each precious stone carries within it clues that scientists can examine to prove the gem's original location. The diamonds, emeralds, rubies and sapphires in a jeweler's cases come in many colors, cuts and settings, but where a gem was mined is not apparent from its physical appearance.

From the startling blues in Brazil's Paraiba tourmalines and India's Kashmir sapphires, to the deep red of Myanmar's Mogok rubies, some regions are especially renowned for their precious stones. In turn, those gems gain cachet, and value.

Rubies and sapphires -- each a form of the mineral corundum -- both offer a considerable amount of visible variation. But that variation is not attributable to a country or region of origin. Scientists must rely on a battery of tests, including microscopic examination and laser-based chemical assessments in order to solve that riddle.

"Over time other deposits may be found in other parts of the world that are equal in beauty but still there's this cultural attachment to Burmese [Myanmar] ruby, Kashmir sapphire, Paraiba tourmaline, things like that that have traditionally been part of the early phases of building the enthusiasm and emotion of a particular material," said George Rossman, a geochemist at the California Institute of Technology in Pasadena, Calif.

Diamonds are beloved, but the verification of that gem's place of origin, or provenance, is often more difficult to address than the origin of other types of precious gems. Rossman said that laboratories have not yet developed "techniques that could work reliably on a single diamond." Governments and industry instituted a tracking system called the Kimberley process to protect against the transportations and sale of so-called "blood diamonds," stones mined in areas of political unrest whose sales help finance wars against legitimate governments.

As helpful as tracking can be, scientists can often discern a gem's origin only after subjecting it to various objective tests.

"Some deposits have compositions that are so unique, that you could pick those out with many techniques. But some of them are very, very similar to each other," said Nancy McMillan, a geochemist at New Mexico State University in Las Cruces.

Identification of the origin requires a large library of samples from all over the world; the job of collecting samples is never done, since new deposits and mines pop up regularly. Scientists examine the gems to gather information and in the future compare any gems of unknown origin against the library.

"We almost always start with a microscope because that gives us an idea of what kind of environment the gemstone formed in and whether it's magmatic origin or metamorphic origin," said Shane McClure, the director of identification services at the Gemological Institute of America laboratory in Carlsbad, Calif.

Along with looking at the gem under magnification, scientists rely on a number of other techniques to characterize it. They examine the little bits of other minerals captured inside a gem, called inclusions, the chemical variability within the gem, and its fluorescence -- how it shines under certain types of lasers or ultraviolet light.

The most advanced techniques examine the chemical makeup of the gem, element by element. Scientists use lasers to transform a miniscule amount of the gem into plasma, and then analyze the individual elements within the plasma. One of these methods is called laser ablation inductively coupled mass spectrometry, or ICPMS.

"This technology is extremely sensitive for trace elements; it's becoming one of the methods of choice for doing provenance studies," said Rossman.

However, McMillan, even though she allows that ICPMS "is the most accurate and most precise geochemical analytical tool known to mankind," favors a different technique, called laser induced breakdown spectroscopy, or LIBS. This technique also shines a laser at a sample, producing gas and a spectrum of light. Analyzing that spectrum provides a dizzying 14,000 pieces of information with each shot -- much more than ICPMS -- said McMillan.

"We've analyzed rubies and sapphires from, I think, 21 locations," said McMillan. "Using our method, we can tell at 95 percent assurance where a specimen is from, which is fantastic in my humble opinion."

McMillan also said she believes that LIBS can be used to identify the provenance of diamonds, although much work remains to prove that the technique works sufficiently well.

Rossman said that he uses both instruments, but finds ICPMS "more versatile," especially when the concentration of certain elements is very low. McClure said that LIBS can be too destructive of the sample gem. Both methods can detect minute amounts of elements within a sample.

In 2003, some sapphires on the market that had been treated with beryllium in order to change their color, said McClure. He said that ICPMS can detect that element at levels of about .02 parts per million in sapphires, and is much better equipped than LIBS to address this type of issue.

Despite the many sophisticated techniques at scientists' disposal, the provenance of some gem specimens can be difficult to identify, said McClure.

"Our end goal is to protect the public and what the public buys in the gemstone industry," said McClure.  "We take that responsibility very seriously."

-Chris Gorski, Inside Science News Service 

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