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In this chapter, characteristic transformations of impurity-defect structure of diamonds during HPHT treatment are discussed. Detailed consideration of the processes of nitrogen defect aggregation and dissociation is given. Stimulating action of vacancies and nickel in the nitrogen-defect transformation processes is discussed. It is shown that the understanding of the nitrogen defect transformation processes is a key to recognition of HPHT-treated diamonds. Color transformations of natural diamonds of different types during HPHT treatment are discussed in detail. Special attention is paid to discoloration of brown diamonds and achievement of red/pink colors. It is shown that the transformations of the nitrogen-related defects and vacancy aggregates are the major processes determining the color change in HPHT-treated diamonds. Typical defect transformations during commercial HPHT treatment are discussed.

In this chapter, major structural features and macroscopic defects of HPHT-treated diamonds are considered. The most prominent of these are graphitized inclusions, cracks, surface modification and areas of internal stress. Behavior of inclusions during HPHT treatment and their impact on the clarity of HPHT-treated diamonds is discussed. It is shown that in most cases the HPHT-induced structural features of treated diamonds are similar to those observed in natural untreated diamonds and alone they cannot be used as reliable indicators of treatment.