Vacancy clusters, dislocations and brown colouration in diamond

Following on from the idea that clusters of vacancies are the origin of the featureless absorption and brown colouration in natural diamond, dislocations are shown to exhibit sub-bandgap absorption also. The vacancy cluster idea has arisen from theoretical predictions of π-bonded chains reconstructing the cluster surfaces and has been confirmed by energy loss studies. In contrast, bandgap states at dislocations are observed in brown and colourless diamonds alike, giving rise to weak absorption, which resembles that theoretically predicted from shuffle dislocation segments. This, however, would not account for the degrees of brownness in the diamonds, but it suggests that if such shuffle segments exist, vacancies must have been present and moved to dislocations to create these configurations in the first place. The question arises, what happens to the vast number of vacancy clusters upon high pressure high temperature (HPHT) annealing, which renders the diamonds colourless. Our observations on natural brown diamonds after HPHT treatment suggest that vacancy clusters, trapped in the strain fields of dislocations, grow in size accompanied by a decrease in their numbers; this leads to much reduced optical absorption.