Most people know precious gemstones by their appearances. An emerald flashes deep green, a ruby seems to hold a red fire inside, and a diamond sparkles like a star. It’s more difficult to tell where the gem was mined, since a diamond from Australia or Arkansas may appear identical to one from the Democratic Republic of the Congo.
Beneath the surface of a gemstone, on the tiny level of atoms and molecules, lie clues to its origin. At this year’s meeting of the Geological Society of America in Minneapolis, Catherine McManus reported on a technique that uses lasers to unravel these clues and identify a stone’s homeland. McManus directs scientific research at Materialytics, in Killeen, Texas. The company is developing the technique.
“With enough data, we could identify which country, which mining region, even the individual mine a mineral comes from,” McManus told Science News.
Some gemstones, including many diamonds, come from war-torn countries. Sales of those “blood minerals” may fund violent civil wars where innocent people are injured or killed. In an effort to reduce the trade in blood minerals, the U.S. government passed legislation in July 2010 that requires companies that sell gemstones to determine the origins of their stones.
To figure out where gems come from, McManus and her team focus a powerful laser on a small sample of the mineral. The technique is called laser-induced breakdown spectroscopy. Just as heat can turn ice into water or water into steam, energy from the laser changes the state of matter of the mineral. The laser changes a miniscule portion of the gemstone into plasma, a gaseous state of matter in which tiny particles called electrons separate from atoms.
The plasma, which is superhot, produces a light pattern. (The science of analyzing this kind of light pattern is called spectroscopy.) Different elements produce different patterns, but McManus and her team say that gemstones from the same area produce similar patterns. Materialytics has already collected patterns from thousands of gemstones, including more than 200 from diamonds. They can compare the light pattern from an unknown gemstone to patterns they do know and look for a match. The light pattern acts like a signature, telling the researchers the origin of the gemstone.
In a small test, the laser technique correctly identified the origins of 95 out of every 100 diamonds. For gemstones like emeralds and rubies, the technique proved successful for 98 out of every 100 stones. The scientists need to collect and analyze more samples, including those from war-torn countries, before the tool is ready for commercial use.
Scientists like Barbara Dutrow, a mineralogist from Louisiana State University in Baton Rouge, find the technique exciting. “This is a fundamentally new tool that could provide a better fingerprint of a material from a particular locality,” she told Science News.
mineral A solid substance that occurs naturally and is obtained by mining.
spectroscopy The branch of science concerned with the investigation and measurement of light patterns produced when matter interacts with or emits electromagnetic radiation
laser A device that generates an intense beam of single-colored light (or other radiation). A laser’s beam is made of photons produced by excited, or energetic, atoms or molecules. Lasers are used in drilling and cutting, alignment and guidance and in surgery. The word laser comes from an acronym: Light Amplification by Stimulated Emission of Radiation.
plasma A kind of gas, plasmas usually have almost no electric charge and can form at low pressures or at very high temperatures.