Gubelin Finds Solution to HPHT Identification

August 28, 2000

Gubelin Finds Solution to HPHT Identification

As a result of a year-long study of about 100 GE/POL diamonds, the Gubelin Gem Lab in Lucerne, Switzerland, says it can distinguish between enhanced and non-enhanced high-color Type IIa diamonds. GE/POL diamonds are enhanced high-color Type IIa diamonds. GGL says it's the first to study GE/POL diamonds before and after the diamonds are treated with a high-pressure/high-temperature process.

"A before-and-after analysis is crucial for understanding the modifications that are taking place when the diamonds are exposed to HPHT conditions," says Christopher P. Smith, GGL'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."

GGL used 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 theories say cause color in diamonds), to detect trace impurities and structural lattice distortion. GGL's most interesting observations involve the crystal structure of diamonds. Past theories focused on large-scale realignment or plastic deformation of the crystal lattice (as a cause for color or its absence). The lab's upcoming report of the study in the Gemological Institute of America's quarterly journal, Gems & Gemology, promises a new level of detail and understanding in this area. Cathodoluminescence, in which visible fluorescence is measured by exposing a diamond to an electron beam, provided a few extra clues. GE/POL diamonds showed blue cathodoluminescence and no phosphorescence. As a point of difference, non-enhanced, high-color Type IIa stones exhibited white, yellow and pink cathodoluminescence.

While GGL's research focused on GE/POL diamonds, the knowledge gained has also been applied to other diamond types in which color is being enhanced. "It is a tremendous foundation for future analysis of HPHT treatments," says George Bosshart, GGL's chief gemologist. Smith and Bosshart say diamond presses used in the growth of synthetic diamonds could also be used to enhance the color of natural diamonds. They say HPHT enhancements of high-color Type IIa diamonds are being sold commercially from sources such as Russia, Sweden, Israel and China. "We have the first verifiable evidence of this Type IIa material, treated in Russia, entering the market without disclosure," says Smith.

GGL warns no single gemological property or characteristic unequivocally identifies all diamonds whose color grade has been improved by the HPHT process. Instead, a combination of sophisticated tests and observations need to be performed. Earlier this year, separate studies by De Beers and the SSEF Swiss Gemmological Institute, Basel, Switzerland, researched a method of identifying HPHT-enhanced Type IIa diamonds through analysis of spectroscopic peaks (click here for this story). But GGL says spectroscopic peak testing alone cannot be used definitively. GGL findings from a larger pool of diamonds studied showed overlaps were recorded between the characteristics of enhanced and non-enhanced high-color diamonds.

- by Robert Weldon, G.G.