REBECCA L. RAWLS, C&EN WASHINGTON

For all their glamor, sparkle, and beauty, there's a darker side to diamonds. In several African countries, including Sierra Leone, Angola, and the Democratic Republic of the Congo, civil wars are being financed in large part by the sale of diamonds coming from regions under the control of rebel groups.

Since May of last year, an international effort has been under way to eliminate world trade in "conflict diamonds," as stones from these regions are called. Known as the Kimberley process, after the location in South Africa where the first meetings were held, these talks enter their third round this week in Namibia. Representatives from the U.S. and most of the other major diamond exporting and importing countries will attempt to develop technical standards for a certification process that can verify the provenance of diamonds in world markets.

It's not an easy task. As a White House conference on the subject last month made clear, it's practically impossible to examine a diamond and tell where it came from. And although spectroscopic techniques, chemical analysis, and laser or ion-beam tagging technologies may eventually have a role to play in helping to certify where a diamond was mined, near-term solutions are much more likely to depend on sealed containers and certificates of authenticity of the sort used to authenticate evidence in court proceedings.

Diamonds are almost pure carbon and have a very limited range of chemical variation, conference participants noted. Some contain small inclusions of minerals such as garnets. But even when present, these materials provide information about conditions in Earth's mantle where diamonds form, rather than about where they were mined, James E. Shigley, director of research at the Gemological Institute of America, told the conference.

In a few cases, he noted, diamonds from a particular deposit have similarities in their shape and growth features. But even then, not every individual diamond will fit the pattern, making it quite easy to slip a few contraband diamonds into shipments of such diamonds without being detected.

ANOTHER COMPLICATION is that most conflict diamonds are alluvial diamonds. After they were brought up from deep within Earth by volcanoes, rivers washed them to the Atlantic Ocean where they settled out in sediments that have now become part of the African continent. It's these sedimentary deposits that are being mined to recover the diamonds.

Little research has been done on alluvial diamonds, and one conference recommendation was to gather much more data on diamonds of this type. In fact, the conference urged that a much larger collection of diamonds from known locations be made available for study in order to look for connections between the physical and chemical properties of these stones and where they were mined.

Although most efforts to use chemistry to trace diamonds to specific mines have been unsuccessful, one preliminary study conducted in the early 1990s hinted that diamonds from Sierra Leone may have an unusual ratio of sulfur isotopes in mineral inclusions found in the diamonds. The isotope ratio can be determined using secondary ion mass spectrometry.

However, as Lawrence A. Taylor, director of the Planetary Geosciences Institute at the University of Tennessee, Knoxville, pointed out at the conference, the differences found in this study are based on data from only a handful of Sierra Leone diamonds, too few for the findings to be definitive. Nevertheless, such isotopic differences "have good potential for a means of discriminating diamonds from this geographic region," he suggested.

MORE PROMISING, perhaps, are preliminary data from recent experiments described by mineralogy professor George R. Rossman of California Institute of Technology. In studies conducted just days before the White House conference, Rossman and his colleagues examined uncut alluvial diamonds under a Raman microscope and found contaminants lodged in the crannies of their surfaces that appear to be of geologic origin. The Raman spectra indicate that the materials are silicate and carbonate minerals that could be residual dirt from where the diamonds were mined.

In what Rossman calls a proof-of-concept experiment, the Caltech team heated a sample diamond from Botswana in a microfurnace, collected the volatiles that came off at different temperatures, and analyzed the hydrogen isotope ratios in the volatile fractions. Although Botswana is not a conflict region, this country in subtropical Africa provided a test location for the study. Rainwater from different regions of the planet contains different ratios of deuterium to hydrogen, Rossman notes. When the researchers examined the material that volatilized at the appropriate temperature to represent water being released from clay minerals, they found that it contained a subtropical isotopic signature.

The results suggest that local rainwater may become incorporated into minerals like clays which, in turn, lodge in crannies on the surfaces of diamonds. "If it turns out, with luck, that these waters, once incorporated, do not readily exchange, then we have locked in a sample of the geography, so to speak, in which these diamonds settled," Rossman explains.

It takes sensitive instruments to detect the sort of isotopic differences the Caltech scientists are measuring, so the technique is not likely to be practical for routine screening of the millions of diamonds that are sold each year. Its potential, Rossman suggests, lies more as a court of last resort when, for example, someone questions the authenticity of the certification on a particular batch of diamonds.

"There's a lot more research that needs to be done," he notes. "We have to get standard diamonds from known localities. We have to test the methods. We have to look at diamonds that have been transported down river systems and compare them to others that were mined directly where they were erupted."

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