High Pressure High Temperature
- Nature of gemstone: This technique is applied to diamonds.
- Possible changes: Type 2a high colour (brown) diamonds to colourless or near colourless. Type 1a brownish colour diamonds to yellowish green.
- Conditions: High temperatures (1600°C - 2000°C) and high pressures up to 70,000 atmospheric pressure) for several hours to days. These are the conditions required for diamond to crystallize whether naturally or synthetically.
- Cause: The cause of colour in diamond is the vacancies and defects. When diamond is exposed to HPHT conditions, the crystal lattice undergoes through the process of reconfiguration, which results in the destruction of vacancies thereby changing the colour of the stone.
- Identification: The conclusive identification of these treated stones is very difficult or even impossible with classical instruments. Sometimes, identification can be made if internal features like graining and strain patterns are observed. Graphitization along cleavage cracks or feathers, melted crystals and burst halos are some other features that may be encountered in HPHT treated stones. A conclusive identification can be made with photoluminescence or cathodoluminescence spectra. They contain an almost negligible nitrogen percentage which is observed in the spectra.
These treated diamonds were launched in 1999 A.D. by Lazare Kaplan International (LKI) and General Electric (GE). These diamonds were marketed with a branding on the girdle GEPOL to represent General Electric and Pegasus Overseas Ltd. (the marketing firm). In 2000 A.D., they were marketed under the name of Bellataire diamonds.