Much progress has been made in recent years towards greater understanding of the various point and extended defects that give rise to colour in diamond and how such defects can be modified via treatment to change a diamond's colour. Such fundamental understanding has been vital in providing reliable means for gemmological laboratories to identify treated diamonds and ensure maintenance of consumer confidence in diamond. The application of high pressure high temperature treatment to remove a brown colour component as prompted significant research recently into the cause of brown colour. This article reviews progress to date and the evidence that indicates that clusters of around 60 vacancies are the defects responsible for the absorption causing brown colour. The general principles and techniques applied to the detection of diamonds subjected to high pressure high temperature treatment are also discussed. Detailed comparison between photoluminescence spectra from treated and untreated diamonds is vital in the identification of Type IIa diamonds, whilst absorption features provide a robust means of identification for Type I diamonds. A detection scheme based on the shape of the platelet-related infrared absorption is presented.

Keywords: Diamond   High pressure high temperature treatment  Plastic deformation  Absorption spectroscopy