Gems of war: scientists struggle to identify conflict diamonds

Science News,  August 10, 2002  by John Pickrell

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Diamonds, in contrast, are relatively pure, and all are created under similar conditions. Identifiable impurities crop up in parts per million, per billion, or even smaller concentrations, making it next to impossible to distinguish among the gems' geographic sources, says Heaney. The more valuable the diamond is, the harder it is to identify.

"Consumers want diamonds to be pure with no ... imperfections," he notes, but these are exactly the characteristics that might help mineralogists determine the gems' origin.

Scientists haven't let this paradox dissuade them from searching for other types of diamond fingerprints. Since diamonds are almost entirely carbon, some attempts to fingerprint them have examined ratios of different types of carbon atoms, or isotopes. Carbon atoms typically have six protons and six neutrons, but isotopes occasionally crop up that have seven neutrons. Therefore, some scientists have postulated that diamonds from a particular location might have a characteristic ratio of the two types of carbon isotopes. Unfortunately, says Heaney, "in reality, there is very little trend in isotope ratios" from one diamond-mining region to another.

Researchers are also looking at nitrogen, the most common impurity in diamonds. This element is structurally similar to carbon and sometimes slips into a diamond's structure. Again, says Heaney, there appears to be little pattern in nitrogen abundance between mining regions.

A different method looks not to the diamond structure but to tiny mineral grains, known as inclusions, that get trapped within the diamond as it grows. James Farquhar of the University of Maryland in College Park and his colleagues announced at the recent AGU meeting that diamonds formed during different geological periods have variable abundances of rare sulfur isotopes within these inclusions. The scientists managed to find specific sulfur-isotope ratios common to eight diamonds known to be from a single mine in Botswana yet not to the other diamonds tested. The researchers, however, have yet to apply the technique to diamonds from any conflict zone.

ROCKY RARITY Heaney and Vicenzi both work with rare diamonds known as carbonados, which are found only in Brazil and central Africa. Carbonados aren't of gem quality. Unlike their more valuable brethren, each stone consists of multiple crystals, is black or gray, and has many imperfections. Techniques that reveal these imperfections may eventually be applicable to more costly diamonds.

Imperfections in carbonados manifest themselves as missing carbon atoms or entire missing layers in a crystal structure. New methods show that these same characteristics sometimes turn up in ordinary diamonds. The defects may be the key to tying any diamond to its source, says Heaney.

At the AGU meeting, Heaney discussed his work comparing defects in carbonados acquired from two sources. His team used a high-energy beam of gallium ions to shear thin slices from stones and then examined the slices under a powerful electron microscope. The frequency of types of defects differed between samples from the two sources.