Parameters of HPHT Treatment

Abstract

In this chapter the main parameters of HPHT treatment are discussed. They are temperature, pressure and time. Temperature of HPHT annealing is the most important parameter, which determines the rate of the defect transformations in the diamond lattice and, consequently, the final structure of the treated diamonds. The range of temperature used for HPHT treatment is discussed in detail. The influence of pressure on the process of defect transformation during HPHT treatment is discussed. It is shown that pressure during HPHT annealing may vary considerably yet remaining in the range of graphite-diamond phase transition. Since kinetic of many defect transformations during HPHT annealing is slow, the duration of treatment is taken into account as an important parameter too. The differences of long-term and short-term HPHT treatments are discussed. High temperature short-term HPHT annealing is shown to be the most effective way of commercial HPHT treatment. Temperature–pressure-time parameters of HPHT treatment are compared with these of natural HPHT annealing the natural diamonds experience in earth. Color transformations in diamonds of various initial colors HPHT-treated at different temperature/pressure/time parameters are discussed. A particular attention is paid to the color transformation of brown diamonds. Optimal parameters of removal of brown color are discussed.