Professional Jeweler Archive: Gubelin Solves HPHT Identification

November 2000

Diamonds/Gemology


Gübelin Solves HPHT Identification

Swiss lab defines a combination of techniques to conclusively identify high-pressure/high-temperature-treated diamonds


After a year-long study of about 100 GE/POL diamonds, the Gübelin Gem Lab in Lucerne, Switzerland, says it can conclusively distinguish between high-color Type IIa diamonds that have been treated with high pressure and high temperature and those that haven’t.

The best-known HPHT diamonds are GE/POL (see “About GE/POL” on p. 25), though comparable ones are entering the market from other sources also.

The Gübelin lab says no single gemological property or characteristic unequivocally identifies all HPHT-treated diamonds. Instead, it says, a combination of sophisticated tests and observations needs to be performed.

Before and After

Gübelin is the first lab to study GE/POL diamonds before and after they were submitted to General Electric for HPHT treatment. “A before-and-after analysis is crucial for understanding the modifications taking place when the diamonds are exposed to HPHT conditions,” says Christopher P. Smith, the lab’s managing director. “It’s then also possible to establish a better means to distinguish between non-processed high-color Type IIa diamonds and GE/POL diamonds of comparable color,” he says.

Past Research

De Beers scientists previously identified tests to detect HPHT treatments, focusing on differences in spectroscopic analysis and on ratios between spectroscopic peaks. (A report on De Beers’ findings was published in the Spring 2000 Gems & Gemology, GIA’s quarterly journal.)

The SSEF Swiss Gemmological Institute, meanwhile, published spectroscopic research describing a method of identifying HPHT diamonds via a peak height ratio of the 637nm N-V and 575nm N-V features (Professional Jeweler, August 2000, p. 40). SSEF says a ratio higher than 2.8 indicates HPHT treatment, while ratios lower than 1.6 are associated with untreated diamonds.

However, Gübelin researchers say spectroscopic analysis alone isn’t definitive because they found overlaps between HPHT-treated and non-treated high-color (D-G) Type IIa diamonds.

Gübelin doesn’t plan to offer HPHT identification service, though it routinely checks all diamonds for clues of HPHT. Details of Gübelin’s research will be published in an upcoming Gems & Gemology.

Modus Operandi

The Gübelin lab began its work by investigating the many causes of color in diamond, including the presence of impurities such as nitrogen and boron. This area was already well-researched, so Gübelin applied these concepts to its investigative procedure using its own diamonds (particularly Type IIa, bought from a variety of sources).

In addition, GE granted Gübelin the unprecedented opportunity to analyze diamonds before and after the treatment. “This allowed us to track what was taking place and clearly identify the changes,” says Smith.

Basic gemological analysis revealed:

  • Overall inclusion features showed little or no change after HPHT.
  • No apparent healing of fractures. In fact, fractures were extended during the treatment in some cases. “The extensions resemble fringes along the boundaries of existing fractures and do not always follow the cleavage planes,” says George Bosshart, GGL’s chief gemologist.
  • GE/POL diamonds show a high percentage of prominent graining, but this doesn’t provide enough evidence for identification because non-processed diamonds can have graining also.
  • HPHT and non-treated Type IIa diamonds show lamellar and tatami-effect graining under cross-polaroid filters. A small percentage of HPHT diamonds show much more intense interference colors with a cellular formation. These filters also bring out strain patterns that are not seen in diamonds without graining. Strain-pattern tests alone are not considered conclusive of HPHT treatment, though the cellular formation may be.

Sophisticated Technology

In addition to basic gemological procedures, Gübelin researchers measured photoluminescence using a Raman spectrometer in conjunction with a variety of lasers. (This allows labs to identify defects that emit luminescence.) A variety of lasers and excitations is better than just one laser at resolving different features in the electromagnetic spectrum.

The diamonds were tested at room temperature and in the cooler temperatures of liquid nitrogen and helium. Among the lasers used: a frequency-doubled argon laser that provided 244 nanometer excitation, a helium/cadmium laser (325nm) and an argon ion laser (514nm).

The lab used this spectroscopic technology in conjunction with cathodoluminescence tests and section X-ray topography tests to study defect centers in the crystal lattice of diamonds (which some theorize cause color). The goal was to detect trace impurities and identify structural lattice distortion. Cathodoluminescence, in which visible fluorescence is measured upon exposing a diamond to a cathode ray, provided a few extra clues. As previously reported, GE/POL diamonds show blue cathodoluminescence and no phosphorescence.

Gübelin’s most interesting observations involve the crystal structure of diamonds. One theory suggested lower colors resulted from large-scale misalignment of the crystal lattice when diamonds were thrust toward the earth’s surface during geothermal activity. Realigning the crystal lattice with HPHT, the theory continued, restored the diamond’s higher color.

However, Gübelin researchers say X-ray topography shows that smaller changes – namely the reconfiguration of point defects attached to glide planes – are the areas where dramatic visual changes occur.

While the lab’s research focused mainly on GE/POL Type IIa diamonds, the knowledge gained has been applied to other diamond types.

  • Gübelin Gem Lab, Lucerne, Switzerland; (41-41) 429-1717, www.Gubelinlab.com.
  • Gems & Gemology, Carlsbad, CA; (800) 421-7250, ext. 7142. (Gübelin study details will be published in an upcoming issue of the journal.)

– by Robert Weldon, G.G.

About GE/POL

Pegasus Overseas Ltd., a subsidiary of Lazare Kaplan International, New York City, announced in March 1999 it would market a small quantity of Type IIa diamonds subjected to a proprietary process developed by General Electric to improve color grades and brightness.

When a furor arose because gemologists couldn’t detect the treatment, the partners agreed to laser-inscribe each diamond’s girdle with “GE/POL.” But the industry clamored for independent identification because the inscription can be polished away and the diamond passed off as natural.

GE subsequently announced it uses high pressure and high temperature to improve the diamonds’ color. Other producers have begun to treat diamonds using HPHT also.

The search for a detection method involves many labs, including De Beers in Maidenhead, England; the Gemological Institute of America in Carlsbad, CA; and the SSEF Swiss Gemological Institute, Basel, Switzerland, in addition to the Gübelin Gem Lab, Lucerne, Switzerland.

The GE/POL diamonds are now marketed to consumers as Bellataire Diamonds (see p. 21). For more information visit professionaljeweler.com, click on “archives” and search for GE/POL or Bellataire diamonds.

– R.W.

This natural color, fancy light brown Type IIa diamond was part of Gübelin Gem Lab’s before-and-after study of GE/POL diamonds. Photo courtesy of Gübelin Gem Lab Ltd. The same diamond after HPHT treatment in which the color was removed. Post enhancement, the diamond’s color grade was D. Photo courtesy of Gübelin Gem Lab Ltd.
All GE/POL-treated diamonds exhibit blue cathodoluminescence when exposed to an electron beam. Photos courtesy of Gübelin Gem Lab Ltd.

Copyright © 2001 by Bond Communications