Following the suggestion that clusters of vacancies were the origin of the featureless absorption and brown colouration of natural diamond, dislocations were also shown to exhibit sub-bandgap absorption. The vacancy cluster idea had arisen from theoretical predictions of π-bonded chains reconstructing the cluster surfaces, and this was confirmed by energy-loss studies. However, band-gap states at dislocations were observed in brown and colourless diamonds alike; giving rise to a weak absorption which resembled that theoretically predicted for shuffle dislocation segments. This however could not account for the degrees of brownness in the diamonds, but suggested that, if such shuffle segments existed, vacancies must be present and moved to dislocations to create these configurations in the first place. The question then arose as to what happens to the vast numbers of vacancy clusters upon high-pressure high-temperature annealing; which render the diamonds colourless. The observations of natural brown diamonds after high-pressure high-temperature treatment suggested that vacancy clusters, trapped in the strain fields of dislocations, grew in size accompanied by a decrease in their numbers; this leads to much reduced optical absorption.

Vacancy Clusters, Dislocations and Brown Colouration in Diamond. U.Bangert, R.Barnes, M.H.Gass, A.L.Bleloch, I.S.Godfrey: Journal of Physics - Condensed Matter, 2009, 21[36], 364208