Excerpt from New Scientist: newscientist.com
Colour profile exposes faked or stolen gems - Anna Gosline
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, both in the US,
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.
Ultraviolet “fingerprint”
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, US, last month.
George Rossman, a mineralogist at the California Institute of
Technology, US, is sceptical of the claim that every gem’s
spectrum will be different. But mapping the variations in
colour, he says, could provide useful “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.
Tas