Abstract: High pressure and high temperature (HPHT) intensify the plastic deformation of natural Ⅰa brown diamonds and make the dislocation in the crystal multiply and slippage, thus the brown transforms into the yellow-green. HPHT provide the lattice defects in Ⅱa brown diamonds with sufficient potential energy for movement, recombination and annihilation, and restore them to the stable state and original colourlessness. HPHT-processing of the natural gray diamonds containing graphite and a new model of the colour enhancement are proposed. It is proposed that there occurs a polymorphous transformation from graphite inclusions to diamonds during HPHT-processing. As a result, microscopic graphite inclusions disappear completely, thus removing the gray component and making diamond crystals more transparent (colourless).