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Gemstone Geography: New technique discerns emeralds' beginnings

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9:21am, December 10, 2003
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Once an emerald leaves its country of origin and circulates around the world, the gem's provenance becomes murky. Scientists have now developed a nondestructive method for determining the source of an emerald, even down to the mine from which it was extracted. That information can affect the gem's price and make it easier for historians to reconstruct ancient trade routes.

An emerald-tracking procedure that measures the ratio of two oxygen isotopes in a microscopic sample from a gem has been available for a few years (SN: 3/11/00, p. 175: Available at Where the Gems Are). Unfortunately, that method is not foolproof, says Philippe de Donato of the École Nationale Supérieure de Géologie in Vandoeuvre-lès-Nancy, France. Emeralds from Russia, Pakistan, and Madagascar often have the same ratio of oxygen isotopes, making them indistinguishable from one another.

A new analysis technique focuses on water trapped in an emerald's minute channels, de Donato and his colleagues reported last week at the Materials Research Society meeting in Boston. These channels, distributed throughout the stone, are just wide enough to fit one or two water molecules. The researchers homed in on a naturally occurring form of water in which an atom of deuterium, a doubly heavy isotope of hydrogen, replaces an atom of the more common hydrogen.

In the new technique, de Donato's team shines infrared light on an emerald. Oxygen-deuterium bonds in the gem's water molecules absorb specific wavelengths of the light, yielding an absorption spectrum that serves as an optical signature. The investigators used this signature to link various emeralds with their known sites of origin.

"Because this method is completely nondestructive, we can make all the measurements we want," de Donato says.

Not only could the researchers distinguish between an emerald from Russia and one from Madagascar, they could pinpoint the specific mine in each country from which the emerald came. So far, the scientists have distinguished among emeralds from 10 mines in seven countries.

They have also discriminated between natural emeralds and synthetic ones.

Why water molecules in emeralds from different parts of the world produce different optical signatures is unclear. De Donato says it may have to do with the presence of soil nutrients, such as sodium and potassium, whose concentrations vary from region to region and that seep into an emerald's crystal structure. The proximity of these elements to water in the gem's channels could influence the spectrum, he says.

"This could straighten out a lot of the confusion surrounding where ancient emeralds come from," says Fred Ward, a gemologist and book author in Bethesda, Md. For instance, when Spanish explorers brought emeralds from Colombia to the Middle East in the 16th century, they kept the origins of their gems a secret to protect their sources, Ward says.

The method could also be useful for documenting new emeralds, he says. For example, if gem dealers can confirm that a stone is from Muzo, Colombia, the most famous emerald mine, they can sell it at a premium.

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