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Techniques of Natural & Treated Red, Pink Diamonds
Gemmological Expert Explains Identification
By: Journal of Gem and Jewellery Industry
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Sep 15 2005 12:00AM
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Reference: 42  

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The allure of color and the beauty of diamonds have a powerful appeal for the gem-buying public and demand for fancy-colored natural diamonds has increased sharply in recent years. Their availability, however, has remained very limited, especially in the highly prized pink-to-red color range. Natural color diamonds with a dominant red color are some of the most sought-after and typically achieve very high per-carat prices when they come up at auction. The highest price ever paid per carat for a diamond at auction was the 0.95 ct. Hancock Red, which achieved an extraordinary $924,000 per carat !

Even the expanded supply of pink diamonds from the Argyle mine in Australia has not satisfied the public’s growing appetite for these fancy-colored diamonds, and reds have remained just as scarce and sought-after as ever. As a result, a variety of attempts have been made over the years to impart a pink-to-red color in diamonds. This has ranged from rather low-tech coatings to the application of more sophisticated combinations of treatment techniques involving irradiation with low-energy electrons, followed by annealing. Most recently the application of High-Pressure/High-Temperature (HPHT) technology has provided an additional method to create such color.
Christopher P. Smith, Director of Identification Services at the GIA Gem Laboratory in New York, gave a presentation on the identification of treated-color-pink-to-red diamonds at a conference organized by the Accredited Gemologists Association (AGA) in Las Vegas. The Conference was chaired by Antoinette Matlins, a well known gemmologist and author.
Lucent Diamonds of Denver, a longtime producer of HPHT-grown synthetic diamonds and has dealt extensively with the application of HPHT treatments to modify the color of natural and synthetic diamonds.

Working in partnership with new diamonds of Siberia and their Research and Development Manager, Dr. Victor Vins, Lucent has developed a unique multiple-step procedure to treat natural, type Ia diamonds. The result is colors that range from brown to purple and pink-to-red through HPHT annealing, irradiation, and low pressure annealing at relatively low temperatures. Those diamonds that result in a pink to red color are sold under the trade name Imperial Red Diamonds.
Lucent President Alex Grizenko began the Conference with a briefing on the history and evolution of the treatment process, also covering future production capabilities and marketing plans. Grizenko pointed out that all Lucent-treated diamonds (other colors besides red are produced) will be fully disclosed, including a laser inscription. Grizenko was joined by Vins, for whom he interpreted, in answering question about the technology behind the treatment.

Both Grizenko and Vins described some of the complications involved with their treatment procedure as a result of the HPHT conditions – temperature above 2,510°C and a pressure of 70 kilobars – so Lucent uses primarily higher clarity stones. Those that survive end up with clarity grades in the VS2- SI1 clarity range, said Grizenko. “Color is usually created at the expense of clarity.” He added that the treated diamonds are cut shallow to maximize their color.
Several of the Lucent diamonds were available for viewing by Conference participants, along with natural pink diamonds supplied by Argyle Diamonds, Ltd. Joseph Casella, Argyle’s senior sales executive for Europe and the U.S.; led the team of Argyle representatives at the Conference.
“The process used by Lucent is a stable and permanent treatment,” said Smith, “and no residual radioactivity was detected.” He added, ‘Several standard gemmological properties and characteristics, as well as advanced analytical techniques, will readily identify these diamonds as treated-color, natural-origin diamonds.”
Smith explained the distinguished characteristics of the Lucent Imperial diamonds, based on GIA’s research, and he illustrated with side-by-side photomicrographs and spectra comparing treated-color and natural-color diamonds. These characteristics include etched surfaces, graphitized coating on mineral inclusions, color zoning, strain patterns, fluorescence, visible luminescence and spectroscopy. A detailed discussion of how these characteristics are used to identify these HPHT treated-color diamonds follows :

  • Etched Surfaces :
    Areas around the girdle/pavilion that had not been repolished after treatment showed evidence of severe etching. This is a clear indicator of HPHT treatment, said Smith. “Etch features were also evident in surface-reaching cleavages or fractures,” he noted, adding that etching creates a coarse or granular appearance on the surface of the stones and along fracture walls.
  • Graphitized Coating on Mineral Inclusions :
    “When diamonds are taken to these temperatures and pressures, it is common for the surfaces of the diamond (both external and internal-surrounding included minerals and along fractures) to partially convert to graphite. “The texture, color and appearance of these inclusions are very characteristics,” said Smith. In addition, the conversion to graphite requires about 30 per cent more space, so internally, the resulting pressure it exerts may cause additional stress fractures around included crystals or as extensions of existing fractures”, said Smith.
  • Color Zoning :
    This is especially useful for identification, said Smith. “Color concentrations are one of the most distinctive characteristics of this treatment.” As he showed several illustrations, Smith said, “Purple and pink-to-red color concentrations often occurred with brown concentrations and regions that were near-colorless.” This type of color zoning is not consistent with naturally colored diamonds, Smith said. He also noted that no color concentrations were observed at the culet.
  • Strain Patterns :
    Smith said the most commonly banded strain patterns were observed, showing a low degree of internal strain. Some showed a higher degree of strain, with a combination of banded and mottled patterns, he said, while others revealed a high degree of internal strain.
  • Fluorescence :
    Another very diagnostic feature for the identification of these treated color diamonds was their characteristic fluorescence reaction to both long-wave and short-wave UV light. The Lucent diamonds exhibited chalky fluorescence to long-wave UV, usually in combinations of yellow, green and orange. Under short-wave UV, they showed medium to strong orange fluorescence, commonly with weak yellow zones, Smith said.
  • Visible Luminescence :
    Smith said prominent green visible luminescence was typical, and it resulted from high concentrations of H3 centres (an absorption feature at 503 nm). Orange-red visible luminescence was also seen and was caused by the nitrogen-vacancy defect (N-V) centre positioned at 637 nanometres (nm). Yellow visible luminescence was due to the (N-V)° centre positioned at 575 nm.
  • Spectroscopy :
    In its research, GIA used several spectrometers to identify the occurrence and concentration of various defect centres to identify the presence of the treatment. These included Raman photoluminescence (PL), Infrared and UV/Vis/NIR spectroscopy. Within the infrared region, it was shown that the diamonds were originally type Ia, containing nitrogen in various states of aggrega

The HPHT steps of the treatment process facilitate the de-aggregation of nitrogen defects, resulting in single substitutional nitrogen (Ib component), as well as other defects, while the subsequent high-energy electron irradiation and relatively lower temperature annealing provide a further reconfiguration of defects to produce the necessary color-causing centres, in addition to others that are characteristic of these treatment methods.
Low temperature Raman PL and UV/Vis/NIR spectroscopy further revealed a series of strong absorptions of N-V centres at 637 and 575 nm, 594, H3/H4 at 503, and 496 nm, N3 at 415 nm, as well as others, the combination of which Smith indicated has never been found in natural-color diamonds. Smith said the GIA Gem Laboratory issues a GIA Gemological Identification Report, not a GIA Diamond Grading Report or Diamond Dossier, for Lucent treated-color diamonds.
Following Smith’s presentation, Conference participants examined natural-color and treated-color diamonds using microscopes, a spectroscope and a UV lamp provided by GIA Instruments. Smith, Grizenko, Vins and representatives of Argyle assisted and answered question. All of the diamonds used in the hands-on part of the Conference were provided by Lucent Diamonds and Argyle Diamonds.

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