General Electric (GE) developed the high pressure (HP) – high temperature (HT) colour treatment in the mid-nineties with the goal of changing the colour of 'lower quality' (read: light brown) diamonds. During this treatment, the stone is heated to temperatures of over 2100°C. To prevent the diamond from being transformed into graphite, this takes place at a very high, stabilising pressure. The brown colour in some types of diamonds (including type IIa) is associated with the presence of plastic deformation or defects in the crystal lattice. HPHT treatment changes these defects in the crystal lattice.
Consequently the cause of the colour disappears. The whole process often takes only a few minutes, but the diamond must then be cut to remove the mat, corroded surface and regain its sparkle. The final result of this treatment greatly depends on the initial properties of a diamond and its type. Today a brown, grey or light yellow diamond can be 'transformed' into a diamond ranging from intense yellow to greenish-yellow, pink, blue or colourless.