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De Beers' Research Yields HPHT Secrets
May 6, 2002 10:49 AM
By Dr. Chris Welbourn, and Dr. David Fisher
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With the announcement in March 1999 by General Electric and Lazare Kaplan International (LKI) at color-improved High Pressure-High Temperature (HPHT)-treated diamonds were hitting the market, the major gemological laboratories in Europe and the U.S. launched a surge of research initiatives. In fact, from the fall of 1999 to the winter of 2000, there were articles on HPHT-treated diamonds in each of six consecutive issues of the GIA journal Gems & Gemology.
In an effort to determine how best to identify HPHT-treated diamonds, the De Beers organization has also been very active in researching the HPHT diamond annealing process. This work is a collaboration between the Diamond Trading Company (DTC) Research Centre, Maidenhead, United Kingdom, and De Beers Industrial Diamonds’ Laboratory in Johannesburg, South Africa. The Maidenhead laboratory’s sophisticated spectroscopic equipment and expertise in characterization, combined with the Johannesburg laboratory’s access to industrial diamond synthesis presses and expertise in high-pressure engineering, has led to substantial progress.
Detailed characterization research carried out at the DTC Research Centre and at some of the major gemological laboratories has led to the discovery of a number of features that can indicate HPHT treatment. Techniques include ultraviolet/visible and infrared absorption spectroscopy and, most importantly, laser-excited photoluminescence (PL) spectroscopy, which requires the sample to be cooled with liquid nitrogen to a temperature of –196°C.
The latter technique is particularly important for type II diamonds because it is such a highly sensitive technique. Type II diamonds are very pure and highly sensitive measurements are required to detect the very low concentrations of defects that they contain. Several of the major gem-testing laboratories have invested in this sophisticated PL equipment and are therefore in a position to identify the vast majority of HPHT-treated diamonds.
Inventing the Tools
One of the roles of the DTC Research Centre, in support of the De Beers Gem Defensive Programme (see page 46), is to assist gem-testing laboratories by developing practical instruments to deal with complex identification problems. To this end, a low-cost, easy-to-use instrument, currently in prototype form, is being developed for rapid screening of potentially HPHT- treated type II diamonds (see page 51). This instrument will not replace the sophisticated laboratory equipment but it will greatly reduce the number of stones that will need to be examined in such a time-consuming way.
Another role of the DTC Gem Defensive Programme is to anticipate future developments in treatment techniques (and also diamond synthesis techniques) that could challenge current identification methods, and to research new techniques that will continue to enable reliable identification to take place. Initially, research both by De Beers and the major gemological laboratories concentrated on areas of current commercial concern. In this area, detection criteria have been firmly established.
The initial work has now developed into investigation of wider ranges of starting material, and treatment conditions in which identification may be more subtle. This forward-looking research is currently a very active area for De Beers scientists.
No Magic Box
So where do we stand at present in terms of the practicalities of detection? There certainly is no magic “black box” that can tell an HPHT- treated stone from an untreated stone with 100 percent accuracy. But this would be an unrealistic expectation. Apart from the relatively trivial task of distinguishing diamonds from simulants, no gemological identification task can be achieved with a single, simple, fully automatic technique. The question is, can procedures and, where appropriate, instrumentation be developed that will allow sufficient numbers of stones to be examined cost effectively so as to address the concerns of the trade?
The sophisticated PL equipment that is now in use in the major gem laboratories can identify the vast majority of HPHT- treated diamonds but there will always be a small proportion for which the identification is uncertain. There will also be a small proportion of untreated stones that may be misidentified as treated. With a material as complex and varied as natural diamond such problems are inevitable.
Is the PL equipment so expensive and the measurement so time consuming as to be of little use in practice? It is certainly true that the need to make such measurements has greatly increased the workload of gem laboratories and has led to increased delays in having stones graded. But practical means of screening diamonds to reduce the number that need to be looked at in detail are being developed and implemented in grading laboratories. The fact that laboratories are willing to invest in such equipment demonstrates their determination to achieve the best results possible and so maintain the integrity of natural, untreated diamonds.
Will future developments in treatments invalidate the identification techniques? The honest answer, of course, is that one cannot be sure. But both De Beers and those gem laboratories that carry out research are committing considerable resources to address this issue. Three years ago, HPHT treatment was generally believed to be undetectable. The fact that this is certainly not the case today demonstrates what can be achieved in a relatively short time by dedicated research.
ractical Advice
What practical advice can be offered to the trade to help identify potentially HPHT-treated diamonds? Fancy yellow, greenish-yellow, pink and blue diamonds need to be referred to a suitably equipped gem-testing laboratory. For greenish-yellow diamonds, intense green fluorescence isa good indication that the stone may be HPHT treated but, ultimately, laboratory examination will be required.
Colorless and near-colorless diamonds should first be checked to see if they are transparent to ultraviolet (UV) radiation, and are therefore likely to be type II. This is accomplished using a short-wave UV lamp and a UV- sensitive phosphor — simple equipment that is commercially available. Colorless and near-colorless diamonds that are UV transparent should be referred to a suitably equipped gem-testing laboratory.
What about diamonds that are too small to be cost effectively submitted to a gem-testing laboratory? Colorless and near-colorless stones can still be checked to see if they are UV transparent or opaque. Those that are UV opaque will be type I and therefore unlikely to have been treated. Forthe very small proportion of diamonds that are UV transparent and therefore likely to be type II, the only realistic option is to insist upon a written declaration from the supplier that the stones are natural and untreated. For fancy yellow, greenish-yellow, pink and blue diamonds, the same written declaration should be insisted upon.
The Rules
In the U.S., the Federal Trade Commission Guides §23.22 state that “it is unfair or deceptive to fail to disclose that a gemstone has been treated if … the treatment has a significant effect on the stone’s value. The seller should disclose that the gemstone has been treated. Note to §23.22: The disclosures outlined in this section are applicable to sellers at every level of the trade.”
The rules of the International Jewellery Confederation (CIBJO) state in Article 6.1 of the CIBJO Diamond Book that “if the natural color of a diamond has been artificially altered, it has to be clearly declared as ‘treated,’ ‘artificially colored, or ‘irradiated.’”
In support of this position, the DTC requires all its sightholders to abide by its Best Practice Principles, which state, “We are committed to the highest industry ethics including the following: …full disclosure at all levels of the diamond distribution chain and, most importantly, to consumers, of all treatments to natural diamonds and compliance with rules, regulations and guidelines published from time to time by the diamond industry’s governing bodies.”
How HPHT Works
Brown diamonds, both type I and type II, as part of their geological history have been subjected to mechanical stresses at elevated temperatures while deep within the earth. This causes their crystal lattices to become deformed — a process known as plastic deformation. This process introduces linear defects into the crystal, which are known as dislocations. The brown color is associated with the presence of these dislocations.
When the diamond crystal is subjected to very high temperatures, the structure of the dislocations is modified, causing the brown coloration to be reduced. At these very high temperatures, diamonds will convert to graphite unless very high pressure is applied. This is done using the same kind of equipment as is used for diamond synthesis, e.g., the conventional “belt” presses developed by General Electric, the cubic or prismatic presses used by NovaTech or the BARS presses developed in Russia. Such equipment is complex and relatively expensive.
Typical HPHT conditions can be in excess of 2000ºC and 60,000 atmospheres. Use of such extreme conditions is not without its dangers. Diamonds with inclusions and fractures can be broken and surfaces can become frosted and pitted.
Type I diamonds contain nitrogen as a major impurity, typically present at levels of several hundred parts per million. Type II diamonds are extremely pure, with nitrogen concentrations of less than about one part per million. A very small proportion of type II diamonds are classified as type IIb, due to the presence of trace amounts of boron impurities, which give rise to a blue color. In type IIa diamonds (i.e., those type II diamonds that are not type IIb), if the brown coloration is reduced sufficiently by HPHT treatment, the result is a colorless, near-colorless or sometimes a pink diamond.
Less than 1 percent of natural diamonds are type II of adequate quality to be suitable for HPHT treatment. In brown or brownish-gray type IIb diamonds, which are of course extremely rare, a pale blue color can be produced by HPHT treatment. In brown type I diamonds, HPHT treatment produces particular nitrogen-containing defects which give rise to a fancy yellow to greenish-yellow color, often with strong green fluorescence.
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Tags:
CIBJO, Compliance, Consumers, De Beers, DTC, GIA, Laboratories, Lazare kaplan, Russia, Sightholders, South Africa
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