Researchers develop methods to characterize diamonds

May 29, 2002

Diamonds may be a girl's best friend, but the qualities that make some diamonds so sought after are the same qualities that make them difficult to characterize and fingerprint, according to a Penn State geoscientist.

"The diamond trade and buyers want diamonds to be pure, with no inclusions or imperfections," says Dr. Peter J. Heaney, associate professor of geoscience. "But mineralogists look for inclusions and imperfections to determine the origin of the stones."

Typically, all diamonds have impurities, but they are in the range of parts per million, parts per billion or even smaller. In fact a few hundred parts per million of nitrogen make diamonds yellow, while much smaller amounts of boron will make them blue.

"It takes very sophisticated equipment to measure these things in diamonds," says Heaney. "Using current methods, it is not possible to screen out conflict diamonds because it would be very labor-intensive."

Conflict diamonds are gems marketed by guerillas with the proceeds going to buy weapons. Currently, some diamonds from the Congo basin, Sierra Leone and Angola are considered conflict diamonds. The United Nations has condemned the sale of conflict diamonds.

Diamonds' perfection also makes it impossible to use them as a window into the processes that occur deep in the Earth, because scientists cannot easily trace a flawless stone back to its original mine in South Africa, Canada or Australia. Scientists do not agree on how diamonds are created or where they originate in the Earth. Kimberlite, the material in which gem quality diamonds are found, is much younger than the diamonds it holds, muddying the waters even further.

Heaney and other mineralogists discussed ways to characterize diamonds during two sessions today (May 29) at the Spring meeting of the American Geophysical Union in Washington, D.C. These sessions are a followup to a White House conference held by President Bill Clinton shortly before he left office. At that time, in answer to the question of "How can we find out the geographic origin of a diamond?", the scientists' response was that no economical way existed. Clinton may have been pursuing the problem of conflict diamonds, but researchers have taken the opportunity to explore the more basic science of diamonds as well.

The international diamond industry, including De Beers, the largest diamond producer and distributor, has adopted an agreement supporting, among other things, the use of forgery resistant certificates and tamper-proof containers for shipments of rough diamonds. Hopefully, this Kimberley Process will ensure that only clean diamonds reach the market. However, the Kimberley Process cannot track and identify diamonds that came on the market before the Process was adopted.

From a scientific point of view, researchers are interested in characterizing diamonds because of what that will tell them about plate tectonics, the Earth's formation and processes deep in the Earth. Researchers are currently looking into the crystalline structure of diamonds, chemical composition, isotopic composition, inclusions and other properties to link diamonds to their mine of origin.

Heaney and Edward P. Vicenzi, Smithsonian Institution, used an ion mass spectrometer and a transmission electron microscope to characterize minuscule pieces of diamond removed from gemstones. They can look at the composition and crystalline structure of the diamond without creating a visible scar in the gem.

"Most crystals have structural mistakes such as missing rows or layers," says Heaney. "Different locations may have different defect assemblages so that we can associate diamonds with their original locations."

Except for the slight impurities, diamonds are made up of carbon and are the hardest known material. Various theories of diamond formation suggest that the carbon in diamonds came from methane or carbon dioxide, primordial carbon from the formation of the Earth or organic carbon. Dr. Peter Dienes, professor of geochemistry at Penn State, is looking at the ratios of carbon isotopes in diamonds to determine if the carbon is primordial or organic in nature.

"This characterization may help us understand why plate tectonics began on Earth and shed light on the Earth's past," says Heaney.

Scientists know that diamonds are found only in areas where the oldest rocks are near the surface, never in young areas like the sea floor. To make diamonds, there must be high pressure but only moderate temperatures, because if the temperatures are high, only graphite forms. Thick, old rock formations provide high pressure beneath, but moderate the temperatures creating areas where diamonds may form.

Diamonds are even more mysterious. The brilliant, sparkling gemstones are only half the story. Carbonados, or black diamonds, are apparently formed by a completely different mechanism. While as hard as diamonds, they are not gemstones and are used mostly as an industrial abrasive. Heaney and Vicenzi have studied and characterized carbonados, which are multicrystalline and form in large nodules. While no one knows how carbonados form, the existence of elements that normally oxidize very rapidly, such as iron and copper, indicates the nodules formed in the absence of oxygen. Carbonados, being less valuable than single crystal diamonds, do not figure into the conflict diamond issue, but can shed light on the Earth's structure.

"Besides the conflict diamond issue, there are good scientific reasons to want to be able to characterize diamonds because how diamonds form is still a hotly contested issue," says Heaney.

Penn State

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