Professional Jeweler Archive: The Fingerprint Quandary

February 2001

Diamonds/Gemology


The Fingerprint Quandary

Some seasoned buyers at De Beers say they can identify the sources of some rough diamonds. De Beers scientists are working toward more definitive methods


Conflict diamonds don’t leave war-torn countries with passports or identity cards. If they did, their role in financing the brutal civil wars in Sierra Leone, Angola and the Democratic Republic of Congo would have ended by now. But politicians eager to end those wars are demanding scientists find a way to identify “fingerprints” indicating where a diamond formed so any from conflict zones can be avoided. Is this possible?

“It may be easier to send someone to Mars than to find a feasible way to fingerprint conflict diamonds,” says Craig B. Smith, manager of De Beers’ Geoscience Center in Johannesburg, South Africa. But neither he nor his associate, senior geologist Amanda Quadling, has closed the door on the idea.

From Prospecting to Fingerprinting

“While I’m a pessimist about finding a solution, there may be technology we don’t know about yet,” says Smith. Trace-element research – looking at the morphology of diamonds for elements characteristic of a certain mine, for example –  is one area of concentration for De Beers. But the Geoscience Center’s primary goal is to support prospecting and mining of diamonds, including locating diamond-bearing kimberlite pipes (see “Passengers on a Bus” on facing page).

So the question arises whether scientists can learn more about diamond origin during other research. De Beers has been looking for fingerprints since 1994 to identify stolen diamonds by mine. Success has been elusive, but there are promising forensic possibilities:

  • LA-ICPMS (laser ablation inductively coupled plasma mass spectrometry) – This mouthful of a phrase describes a highly concentrated analysis of a diamond to detect trace elements at a sensitivity of parts per billion. The test blasts minuscule pits into a diamond’s surface. The resulting vapor, or plasma, is fed into a mass spectrometer for analysis. Compositions of trace elements of thorium, uranium or lead can help determine origin. However, diamonds aren’t amorphous. Because of zoning, different parts of a diamond can contain some different elements. Quadling adds that the lack of a big database “leaves us with little knowledge of the concentrations or ratios we should be looking for.” De Beers is refining the test using cathodoluminescence to determine specific parts of a diamond that should be tested. (Cathodoluminescence causes diamonds subjected to a cathode ray to fluoresce, yielding information used in origin studies). LA-ICPMS equipment costs $3 million, and the learning curve to use it is big, say scientists.
  • Sophisticated Gemology – The Geoscience Center uses infrared spectroscopy to determine whether a diamond has nitrogen and, if so, how much. This can suggest a diamond is from one mine vs. another, though overlaps among sources can occur. The center also uses high magnification microscopy to examine inclusions for minerals that indicate origin. But again, there are overlaps.

Eyeballing the Rough

Some De Beers experts say they can determine the origin of diamond rough by examining its surface texture. It’s no doubt a skill learned when De Beers used to buy loose diamonds on the open market and had to make an educated guess about origin. [Editor’s note: De Beers stopped open-market purchases in October 1999 so it could guarantee its diamonds are not from conflict areas.]

In written testimony before the U.S. Congress in June 2000, De Beers said it would help governments interested in the movement of rough diamonds to buy typical mine-run samples to use for comparisons. It also offered the services of about 25 diamond buying experts to teach authorized diamond controllers or customs officials how to distinguish diamonds visually before they are cut.

De Beers even published a pamphlet titled Introduction to the Sources of Rough Diamond Production and Their Individual Characteristics. The pamphlet lists features of diamond rough unique to certain regions (see “Footprints in Carbon,” p. 38). But the descriptions overlap, which makes positive identification of rough difficult.

Similarity in Groups

“Diamonds are like people,” says Charles Skinner, exploration manager for De Beers Angola. “You can’t tell where people are from by just looking at one person. But if you put 20 Americans, 20 Germans and 20 South Africans together in different groups and consider a similarity within each group, then you can identify the groups. Diamonds are much the same way. If you mix up the groups, you have a problem.”

The ultimate question is whether an educated guess about diamond origin will hold up in court. William Boyajian, president of the Gemological Institute of America, seemed to answer that question in his testimony before Congress, saying, “We do not know of any scientific way to determine the country of origin of rough or polished gem diamonds, nor do we foresee practical ways being developed in the near future.”

GIA testimony suggests there are enough loopholes for people on either side of an identity question to be correct technically, particularly in cases where diamonds cross borders naturally through alluvial activity. Consequently, diamond experts believe positive identification is a moot point.

And once a diamond is polished, positive identification becomes even less likely because identifying characteristics can be cut away.


– Robert Weldon, G.G.


At the De Beers Geoscience Center in Johannesburg, South Africa, a scientist examines the microscopic image of laser ablation pits in a mineral on a computer screen at left. The screen on the right is hooked up to a mass spectrometer that measures trace elements in a mineral such as diamond to help determine origin. Photo by Robert Weldon.

 

Passengers on a Bus

Analyzing indicators determines the diamond-bearing potential of a kimberlite pipe

Kimberlite pipes are vertical conduits composed of igneous, peridotitic volcanic rock. They were formed when cratonic pressures gave rise to high temperatures and pressures, then molten rock followed weaknesses in the earth’s mantle and crust to blast through the surface.

The rock in these pipes has indicator minerals and, in some cases, diamonds. Fifty out of every 6,000 kimberlite pipes studied contain enough diamonds to make mining economically feasible. (Less than 1% of pipes studied are feasible to develop – 1 carat per ton of ore is considered rich ore.)

Prospecting involves analyzing kimberlitic samples for indicators such as garnet, ilmenite, chrome diopside, spinel and clinopyroxene. “These minerals could be viewed as passengers on a bus,” says Craig B. Smith of De Beers’ Geoscience Center. Their presence can determine the diamond-bearing potential of a kimberlite pipe. “By examining the nickel concentrations in a garnet, for example, we can determine the heat and pressure it sustained and determine where it was parked in the kimberlite,” he adds. “This helps us calculate whether an area is good to mine.”

– R.W.

Footprints in Carbon

Certain characteristics indicate the origin of a diamond, but some characteristics are found at numerous sources

Here are descriptions of diamonds from some of southern Africa’s main sources, based on De Beers’ pamphlet Introduction to the Sources of Rough Diamond Production and Their Individual Characteristics. These descriptions may help customs officials and authorized diamond controllers spot conflict diamonds, though there are no guarantees. The country is listed first, then the mine if that’s relevant, then the description.

Angola, Lucapa. Mainly octahedral with small cracks at the points. A full range of colors and some frosted stones are found. Note: Some Congo diamonds emanated from the same Angolan source and share characteristics.

Angola, Cuango. Mainly dodecahedral, many with half-moon cracks. Found in a full range of colors, including a large, frosted content. Some Congo diamonds emanated from the same Angolan source and share characteristics.

Botswana, Orapa. A full range of model types, including cubes. A full range of colors, but they tend to be yellow-based.

Botswana, Jwaneng. A full range of models with a high proportion of cubes. A number have a green skin, but the color is lost when the diamond is polished.

Democratic Republic of Congo, MIBA. A high quantity of low-quality diamonds and bort. A large percentage of cubes and heavy, coated stones.

Democratic Republic of Congo, Tshikapa. Crystalline in formation. Alluvial mining. Some Congo diamonds emanated from the same Angolan source and share characteristics. Better overall quality than diamonds from MIBA.

Namibia. Mainly dodecahedrons, many with half-moon cracks. The production has a full range of colors with a frosted content. Namibian diamonds tend to have rounder surfaces, their edges worn down in a 1,000-mile river journey.

South Africa. Kimberlite mining operations tend to produce a whole range of models and colors, many of them smaller. Diamonds from the Kimberley mine tend to be more yellow-based; those from the Premier mine are predominantly brown. South Africa’s alluvial production, in Namaqualand, for example, yields more rounded dodecahedral types.

– R.W.

Copyright © 2001 by Bond Communications