Recent experiments had indicated that the brown coloration was caused by

vacancy-type extended defects. However, the shape and size of these defects

remained unknown. Here, an investigation was made of the structural, electrical

and optical properties of large spherical vacancy clusters, consisting of 71

vacancies, by using density functional theory. A comparison of these calculations

with recent experimental measurements on brown natural diamond revealed

striking similarities and it was therefore suggested that the brown colouration

originated from large spherical vacancy clusters. Large Spherical Vacancy Clusters in Diamond – Origin of the Brown Colouration?

N.Fujita, R.Jones, S.Öberg, P.R.Briddon: Diamond and Related Materials, 2009,

18[5-8], 843-5