While the industry’s primary concern was the transformation of brownish diamonds into colorless stones, a number of yellow, yellow-green, and greenish-yellow High Pressure High Temperature (HPHT) treated diamonds were also introduced. These stones sparked less of a furor because their color was distinctive enough to raise questions, and could be readily identified by a major gemology lab. Soon, however, blue and pink HPHT-treated diamonds began appearing in the market. These colors were of special concern since they corresponded to particularly valuable natural colors.
At the beginning of 1997, diamonds with colors ranging from greenish-yellow to yellow-green were seen on the market, which owe their intensive coloration to another artificial treatment process. Initially, the stones were offered as Russian synthetics, but they turned out to be natural type Ia diamonds whose original brown color was changed by using a special treatment technique operating at High Pressures High Temperatures. The color of these diamonds, is made up of two components: a yellow body color and an intense green luminescence in visible light. Both color and luminescence are clearly zoned. With proper magnification, brownish-yellow graining becomes visible.
An important characteristic used in the detection of HPHT treated diamonds is their fluorescent behavior when exposed to UV radiation. Whereas natural diamonds have a chalky fluorescence, heat-treated diamonds exhibit a mixture of greenish-yellow and blue fluorescence, with intense, green fluorescence regions along the growth lines. Microscopic examination reveals further characteristic properties. They include evidence of burning at facet edges on the crown, fine fissures under the table, and in the girdle region as well as evidence of corrosion on the girdle. Graphitization around inclusions and in fissures may also be observed occasionally.
However, the absorption spectrum is the most striking feature of HPHT treated diamonds. In addition to a 415 nm line and a broad absorption line between 450 nm and 500 nm, which are characteristic of natural greenish-yellow diamonds, distinct absorption maxima are evident at 495 nm and 503 nm-even at room temperature. In contrast to irradiated greenish-yellow diamonds, the typical radiation band at 595 nm is missing. In the near-infrared range, another absorption line (H2 center) is evident at 985 nm. This is regarded as characteristic of stones treated in this way, and it does not occur in natural color stones.
The diamonds described above were treated primarily in Russia. Novatek Inc., located in Utah, appears to use a similar technique. The company has been marketing yellow, yellow-green, and green diamonds under the NovaDiamonds brand since 1999. Orange-yellow to brownish-orange HPHT treated specimens are also observed occasionally. Here again, type Ia diamonds are apparently used as the starting material. They are then exposed to a temperature of around 2000 Celsius and pressure of about 60.000 atm.
Making Diamonds Colorless Using High Pressure High Temperature
In March 1999, another treatment technique to improve color was announced. A high pressure and high temperature process developed by General Electric (GE) made it possible for the first time to make diamonds colorless, or near colorless. Type IIa stones using this method are also known as “Pegasus” diamonds because they were marketed by Pegasus Overseas Ltd. (POL), a subsidiary of Lazare Kaplan International (LKI) based in Antwerp. The name “Monarch TM Diamonds” and today “Bellataire” are also used to market the diamonds. Pointing out that the treatment could not be detected, LKI thought they could sell the treated diamonds without any special identification. As a result of pressure from the diamond trade, and in agreement with GIA, all diamonds treated using this new method required a GIA certificate and had to be marketed with the laser inscription: “GE POL” on the girdle. Pegasus Overseas Limited (POL) states that this diamond has been processed to improve its appearance by General Electric Company (GE). In contrast to the term “processed” according to CIBJO guidelines the diamonds must be clearly identified as treated. The laser inscription is not necessarily an identifying characteristic, because it is placed on the surface and can easily be removed from the girdle by subsequent polishing.
Type IIa natural diamonds range from brownish to brown. They serve as the base material for GE POL diamonds. Professor Dr. Henry A. Hanni, director of the SSEF Swiss Gemological Institute, and his colleagues have succeeded in identifying differentiation characteristics that, since May 2000, have enabled the SSEF Swiss Gemological Institute to reliably detect this treatment in the laboratory and to support the color grading of the Type IIa diamonds.
Only about 2% of all gem diamonds are suitable. GE POL diamonds are identified using a two-stage examination technique. Because Type IIa diamonds are relatively rare, only about 2% of all gem diamonds can be treated using this technique. In fact, up to the end of the year 2000, only about 2000 GE POL diamonds were certified by GIA.
Type II and IIa diamonds in particular can be detected in the laboratory using infrared and UV/VIS spectroscopy. Since they are transparent to short-wavelength UV light (254nm), they can be pre-selected using a short wavelength UV light and the “SSEF Diamond SpotterTM” and DiamondSureTM
The researchers hoped that absorption characteristics would provide important information for use during identification of colorless HPHT-treated diamonds, because these characteristics exist independently of all inscriptions or inclusions. Traditional absorption spectroscopy and infrared spectroscopy have not provided any usable characteristics so far.
These researchers found what they were looking for while studying the photoluminescence spectra of natural and GE POL diamonds. According to the traditional definition, no nitrogen can be detected in Type IIa diamonds, at least not using infra-red spectroscopy. Both untreated and HPHT treated high-color Type IIa diamonds have properties in common which makes it difficult to differentiate between them by routine gemological methods. Hight Pressure High Temperature techniques are advancing rapidly, and enterprises other than General Electric and Lazare Kaplan Int. are already producing enhanced High Pressure Hight Temperature diamonds.
However, Type IIa diamonds can also often be confirmed microscopically. The observation between cross polarization filters shows an anomalous double refraction. This is shown by parallel and cross hatching patterns known as a tatami strain. The treatment as such can be recognized with varieties of lower clarity grades because the inclusions have undergone specific changes. Mineral and graphite inclusions surrounded by small tension cracks are characteristic features in relation to natural, untreated stones. The small tension cracks which expand from the surface of the cut stone towards the inside often contain small graphite particles. As with heat-treated rubies and sapphires, in GE POL diamonds, partially healed cracks and tension fissures can be observed around crystal inclusions. In stones with high clarity grades, the treatment cannot be definitely proven.
GE POL was just the start of an immense wave of new treated diamond products. Hardly a month passes without a new variety of the process being marketed, enriching and bewildering the diamond trade. GE was the first to announce the successful HPHT synthesis of diamonds in 1955.
Hight Pressure Hight Temperature color-enhanced diamonds coming in the USA are not being sold with the same level conscientiousness toward the diamond trade. We know that some of these diamonds are entering the international trade through various channels without any form of proper disclosure. Buyers beware when shopping for diamonds! Look for certification from the well-known Gemological Laboratories (GIA, AGS, HRD, IGI) only.
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