High Pressure High Temperature – A treatment used to improve the color of some diamonds. The treatment is considered permanent and is so similar to natural processes while a diamond was being formed in the earth, that it is very difficult to detect except in the best equipped labs. HPHT treatment should be disclosed by the seller.
According to Wikipedia:
“A small number of otherwise gem-quality stones that possess a brown body color can have their color significantly lightened or altogether removed by HPHT treatment, or improve existing color to a more desirable saturation.
The process was introduced by General Electric in 1999. … Pressure of up to 70,000 atmospheres and temperatures of up to 2,000 °C (3,632°F) are used in HPHT procedure.
Definitive identification of HPHT stones is left to well-equipped gemological laboratories, where Fourier transform spectroscopy (FTIR) and Raman spectroscopy are used to analyze the visible and infrared absorption of suspect diamonds to detect characteristic absorption lines, such as those indicative of exposure to high temperatures. Indicative features seen under the microscope include: partially healed feathers; a hazy appearance; black cracks surrounding inclusions; and a beaded or frosted girdle.
Diamonds treated to remove their color by General Electric are given laser inscriptions on their girdles: these inscriptions read “GE POL”, with “POL” standing for Pegasus Overseas Ltd, a partnered firm. It is possible to polish this inscription away, so its absence cannot be a trusted sign of natural color. Although it is permanent, HPHT treatment should be disclosed to the buyer at the time of sale.”