A kind of atomic birth certificate can peg where emeralds were grubbed from the ground, geologists report  in tomorrow's issue of Science. The technique might help dealers authenticate top-quality stones, and it could clear up the mysterious origins of Old World emeralds, including some famous gems.
When conquistadors in the 16th century captured mines in Colombia, they shipped back chests full of eye-popping emeralds. Even today, dealers have no trouble spotting the exceptional clarity and intense color of the Colombian gems. But it's been difficult to track down the birthplaces of the murkier Old World emeralds. That could change: Over the past few years, geologists have discovered that many emeralds have unique oxygen isotope ratios that depend on where the stones were mined. Intrigued, researchers led by Gaston Giuliani of the Petrographical and Geochemical Research Center (CRPG)-CNRS in Vandoeuvre-lès-Nancy, France, decided to see if they could trace the origins of emeralds in artifacts.
First the researchers had to persuade the relics' wardens that they would do no harm. Reassured that samples weren't visibly marred in a test run, curators let the team have a crack at a handful of gems spanning the history of emerald trading. To measure oxygen isotopes, the researchers fired a beam of cesium atoms at the emerald, vaporizing a few atoms and leaving a hole a mere 20 micrometers wide and a few angstroms deep.
As expected, emeralds from a wrecked galleon bore the isotopic signature of Colombian mines. But surprisingly, an emerald in a Gallo-Roman earring came from the Swat River in Pakistan, demonstrating that the Romans had access to gems from much farther afield than Egypt. And a 13th century French crown, it turns out, is graced by an emerald from the Austrian alps--one that appears to have been unearthed more than 500 years before the presumed source mine came into operation.
"It's a great idea," says Terri Ottaway, a geochemist and gemologist with the Royal Ontario Museum in Toronto, "but I'd like to see it tested with more samples." The technique may eventually help gem dealers tell Colombian emeralds from top Afghani stones or spot synthetic emeralds, which are hard to distinguish from flawless gems. And archaeologists might have an important new tool for tracing stony emeralds, the opacity of which tends to obscure telltale signs of a source region.