What is HPHT technique?
HPHT technique is a complex process of combining high temperatures (at least 900°C, but usually above 1500°C) and high pressures (thousands of atmospheres) on a diamond (or diamond in raw materials). As a result of such processing, a kind of "release" of the diamond occurs - internal stresses disappear, twins dissolve, structural defects change. The result of such transformations is a change in the physical and chemical properties of diamond in particular color which is the main reason for such enhancement. HPHT enhancement is not reversible.
The HPHT treated stones are estimated much cheaper than similar ones (having the same quality properties) unlike untreated ones.