Geotimes
Technology
Fingerprinting a diamond’s source
Naomi Lubick

Diamonds often are not as pure as they look. The glittering gemstones not only have commercial value for jewelry and provide a window into Earth’s mantle processes, but they also fund war in West Africa — through the sale of so-called conflict diamonds from Sierra Leone and elsewhere. Some scientists say that new analytical techniques might hold promise for identifying diamonds’ sources, though many stumbling blocks remain.

In January 2001, the Clinton administration brought scientists together at the White House to discuss possible identification methods, specifically to prevent trade of conflict diamonds. Ideas ranged from encasing legitimately mined raw diamonds in labeled polymer cases to engraving the diamonds themselves with identifying marks (such as a laser-imprinted data matrix; see Geotimes, March 2001).

Most geoscientists think the best currently available method is one that uses an optical microscope. The process leaves no scars and only needs a skilled operator to visually identify any impurities that might be characteristic of a mine, from color to the shape of a raw diamond. For example, some South African diamonds tend to resorb into the mantle, subsequently re-forming with a dodecahedral shape, says Jeff Harris of the University of Glasgow, who specializes in this type of identification.

Still, diamond dealers and others want a more chemically analytical method of identification. Some have suggested analyzing a rough diamond’s surface, to identify provenance from alluvial deposits that might coat it. But diamonds generally are cleaned or cut by the time they make it to a dealer. One even more intrusive method has several research groups drilling deeper to see what impurities might identify a diamond’s source.

That method, ICP-MS, or inductively coupled plasma mass spectrometry, has shown some promise. The technique could identify diamond components by measuring particle masses from very small samples, which is an advantage with diamonds, says Richard Carlson, a geochemist at the Carnegie Institution of Washington’s Department of Terrestrial Magnetism. He works with ICP-MS to determine the isotopic composition of samples from mantle rocks around the world.

New methods using ICP-MS, or inductively coupled plasma mass spectrometry, might have promise for identifying the origins of diamonds, in hopes of preventing sales of “conflict diamonds.” Geochemist Richard Carlson holds a piece of an ICP-MS machine: a small disk with a hole that funnels isotopes to a mass spectrometer. Photo by Naomi Lubick.


The least destructive ICP-MS approach, Carlson says, uses a laser to bore a microns-wide hole in a diamond. The laser ablates a small bit of the diamond into very small particles, which are then swept along into an argon gas plasma. At 8,000 degrees Celsius, the plasma ionizes the particulate matter, producing electrons and positively charged ions of every element in the sample. After passing through two small holes into the vacuum of a mass spectrometer, the ions are accelerated by an electric field and then separated into their constituent isotopes by a magnetic field.

These separated isotope beams can relate the isotopic composition of a sample. A group from Antwerp, Belgium, published positive, though preliminary, results for a series of elements using laser-ablation ICP-MS in the Journal of Analytical Atomic Spectrometry this summer. The results have been met with some skepticism as to the detail of the elements identified, due to the lack of statistical analyses to back it up.

One problem ICP-MS researchers will have to address is the lack of a standard, or a sample that could provide measurements for comparison, in order to calculate the concentration of elements in the target diamond. The diamond industry could make their own diamond standards with fixed levels of particular element impurities, Harris says, which would be no different than synthesizing a diamond. Such a standard is necessary “to take the vagaries out of the machine,” he says, which is particularly important for the raw data that ICP-MS produces.

Even more challenging, Harris says, will be establishing a database that documents diamond compositions and characteristics of specific mines — something difficult to accomplish, as such a catalog will have to include samples from conflict diamond regions. Unfortunately, the illegal trade regularly mixes conflict diamonds with legitimately mined batches, which further thwarts any cataloging process, he says, with the potential to mislead researchers on the composition of diamonds from specific mines.

Additional drawbacks to ICP-MS, Carlson and others note, include its time-consuming procedures, which require well-trained technicians. The method also requires expensive equipment: Multiple ICP-MS machines for assembly-line work would be necessary, at upwards of $100,000 each, and that cost does not begin to account for the argon gas supplies needed on a daily basis. Plus, finicky buyers probably will not want the several-micron-sized holes marring a tested diamond.

The real showstopper, though, could be the lack of large and consistent variations in diamonds. “Diamond is a very clean mineral,” Carlson says. “Part of the reason it’s so strong is that it’s put together very well.” Its stable crystal structures resist change, he says, despite high temperature and pressure changes the minerals might experience as they travel through the mantle. The gemstones are carried to the surface, most often in kimberlite pipes, from around 150 kilometers depth.

But diamonds aren’t completely homogenous either, and some chemical signatures can survive the trip. Nitrogen atoms sometimes fit into a diamond carbon structure. In time, individual nitrogen atoms in a new diamond migrate and cluster, so that older diamonds have randomly distributed nitrogen platelets. “It takes billions of years at mantle temperatures,” Carlson says, but such clustering might point to individual mines as sources.

Nitrogen absorbs infrared light, Carlson says, which can be measured with Fourier Transform Infrared Spectroscopy, or FTIR. The method is promising because it can measure nitrogen absorption without damaging the diamond.

As diamonds pass through the mantle, they also might pick up or form around fluid intrusions that trap distinctive trace element signatures. A non-invasive method to measure these trace elements uses a proton microprobe: An accelerator shoots protons at a diamond sample, producing an X-ray spectra. Combining that process with ICP-MS, Bill Griffin and his co-workers at Macquarie University in Sydney, Australia, found diamonds from one Western Australia field that “were completely different from anything we’d seen,” Griffin says, containing higher calcium concentrations, which may provide a potential tracer element.

But Griffin is dubious that a systematic testing system might work, despite this small success. “It’s hard to imagine that [mantle] processes are going to be sufficiently different from place to place,” he says, so that each individual mine will have a trace element signature. Also, Griffin says, “all these processes will be changing with depth,” perhaps within a single mine, making source identification more challenging.

While geoscientists continue to work on laboratory identification methods, many diamond-mining countries have signed onto the Kimberley Process, which stemmed from agreements in Kimberley, South Africa, in 2000, and officially was inaugurated last January (Geotimes online, November 2002). The certification scheme established under the agreement follows rough diamonds, requiring dealers to obtain and display proof of where the diamonds were originally mined. It seems to be working. Harris says that about 2 percent of world production is conflict diamonds, down from 4.5 percent in 2001, when scientists first gathered at the White House for brainstorming.

Nevertheless, that’s not enough for Harris: “The conflict diamond question is still with us,” he says. Harris distributes parcels of diamonds for the DeBeers diamond company to geoscientists for testing new techniques, and lately he has been providing diamonds for research groups around the world. “There’s a beginning,” he says, in pursuing whether “the provenance of a diamond can be determined using sophisticated analytical tools.”


Lubick is a staff writer for Geotimes.

Links:
"Conflicted Stones," News Note, Geotimes, March 2001
"Governments agree to stop conflict diamonds," Geotimes online, Nov. 22, 2002

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