High-pressure high-temperature (HPHT) treatments are used to improve the color grade of diamonds. It was first used in the 1970s to produce yellow and green diamonds.

Today, the HPHT process is used to turn cheap brown diamonds into more expensive colorless diamonds. General Electric has patented a variation on the technique - such diamonds are sold as Bellataire ® diamonds.

In the case of colorless diamonds, this treatment is effective only on very rare Type IIa or Type Ia/b diamonds.

The HPHT process is a form of diamond treatment, so such diamonds should be disclosed to the buyer. Diamonds treated in this manner are becoming more widespread, as it allows lesser color grades of diamonds to be sold at higher prices.

Detecting diamonds treated by the HPHT process takes special equipment and skills. The best way is to obtain a report from a gem lab. If the report states that the diamond is "enhanced", it means the diamond has been treated.