Colour profile exposes stolen gems

03/09/05

JEWELLERS say that every gemstone is unique, and soon they may be able to prove it. That will make it easier to recover lost or stolen gems, even after they are cut or altered.

The jewellery industry has long looked for ways to establish a gemstone's type and quality, and to spot fakes. But identifying individual stones is difficult because there are so many, and the good ones are quite similar.

Now a technique dubbed "microspectrometry" could change that, by mapping the unique colour patterns of each gem. Mike Eyring of the Arizona Department of Public Safety and Paul Martin of CRAIC Technologies in Altadena, California, measured the spectra of the ultraviolet and visible light absorbed by three sapphires and three lower-value red stones called spinels, often used in place of rubies. Sapphires get their blue colour from charge transfer between iron ions. Spinels, like rubies, glimmer red from chromium impurities. The researchers recorded the spectrum emitted by a 10-micrometre-wide point on each stone's surface and found that every one, even those of the same type, had a different spectrum. The differences were particularly marked in the ultraviolet range. "That could be your fingerprint," says Martin, who presented his results at the American Academy of Forensic Sciences meeting in New Orleans last month. George Rossman, a mineralogist at the California Institute of Technology, is sceptical of the claim that every gem's spectrum will be different. But mapping the variations in colour, he says, could provide useful information. "The question that needs to be researched is whether this tool adds enough to what is out there already," he says.

The Gemological Institute of America, for example, grades gemstones by generating maps of each one's flaws, as well as recording their cut, carat weight and clarity. Every measurement is added to a database, and stolen stones have often been identified after being resubmitted for grading.

Source: Eurekalert & others

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