An HPHT stand for High Pressure High Temperature, which is very similar to nature treatment, is a process which is very similar to nature, putting the diamond back into the environment in which is formed.

HPHT used to transform some unpopular brownish into more expensive colorless diamonds but still very few diamonds qualify for this process.
The HPHT process is absolutely permanent and irreversible. There is no need to avoid extreme temperatures as with other color treated diamonds. Because the process is permanent, it is possible to obtain GIA or EGL certificates for HPHT-treated stones.

HPHT-treated diamonds offer excellent value, as they are of high color and clarity but are less expensive than untreated stones.