Where the Gems Are

Article excerpt

Technique adds new facet to emeralds' origins

The brilliant, deep--blue-green emeralds that come from the mines of Colombia outshine their Austrian and Zambian cousins in both beauty and value. For an emerald, a Colombian pedigree confers not only spectacular color but also eye-popping prices. Both historians and gem dealers have a stake in figuring out an emerald's origins.

Now, researchers in France have found a novel way to track down the birth records of these precious stones. By measuring the ratio of two different isotopes of oxygen in the gems, they can pinpoint an emerald's source--down to the exact mine. Combined with traditional gemology techniques, the oxygen-isotope analysis could bring new insights to the history and formation of these coveted gems.

Emeralds are a kind of beryl--beryllium aluminum silicate with a dash of chromium, which produces the characteristic green. The primary method of distinguishing one emerald from another is by looking at the gem's inclusions, microscopic cavities within the crystal. If the inclusions contain gas, liquid, and solid components, says Fred Ward, a gemologist, author, and photographer from Bethesda, Md., "you're 99 percent sure it's a Colombian emerald."

Emeralds of excellent quality, however, don't have many inclusions, often making it hard to settle disputes over sources.

A team of scientists led by Gaston Giuliani of the Petrographic and Geochemical Research Center in Vandoeuvre-les-Nancy, France, discovered several years ago that emeralds from different mines possess unique isotopic signatures. "There was a big difference in the data between the oxygen isotopes of [emeralds from] Brazil and Colombia," says Giuliani. "This was a result of the different geology of the two sites."

The researchers systematically catalogued the isotopic ratios of emeralds collected from different mines around the world. Using this information, they analyzed the origins of nine emeralds of historical interest. In the Jan. 28 SCIENCE, they report that they were able to confirm the sources of many of the gems and learn some surprising things about them as well.

The test uses an instrument called an ion microprobe, which blasts a few atoms from the gem's surface. The damage is far too small to be detectable.

Four of the emeralds belong to the treasury of the Nizam of Hyderabad in India. According to folklore, the gems had been unearthed from long-lost Indian mines. The oxygen-isotope analysis revealed that three of the emeralds came from Colombia--each from a different mine--and one originated in Afghanistan. …