High-Pressure, High-Temperature (HPHT) is a diamond treatment in which natural brown diamonds are heated to improve their color. Nearly all “white” diamonds contain some degree of brown or yellow color.
The HPHT technique can be applied to diamonds that contain nitrogen (Type I), which turns the diamond yellow, or to diamonds that are nearly free from nitrogen (Type II), which makes the stones colorless.
While gemologists can detect HPHT-treated diamonds, submitting HPHT diamonds for grading without disclosing the information is a violation of gemological laboratory regulations, which stipulate that diamond traders provide full details about any treatment given to gemstones submitted for grading.
The Senior Vice President of the GIA Laboratory and Research says that the range of diamonds subjected to the HPHT treatment has expanded over recent years though the GIA remains confident that its research into diamond treatment has given it the tools necessary to identify treated stones.
According to De Beers, Type I HPHT-treated diamonds exhibit yellow to brown graining; pitting and burn marks; inclusions surrounded by cracks; graphite at fractures; green fluorescence in visible light and greenish-yellow or yellowish-green fluorescence under UV light. The detection of Type II HPHT-treated diamonds is more difficult, and to that end De Beers developed a special screening device, the DiamondPlus, which measures a stone’s photoluminescence.