Natural type-IIa brown diamonds were found to contain concentrations of trapped monovacancy-type defects and large vacancy clusters which greatly exceeded the vacancy content in natural or synthetic type-IIa colorless diamonds. A proprietary heat-treatment reduced the vacancy cluster content by about 50%, and removed the brown coloration. The origin of the brown coloration was associated with highly distorted regions.

Vacancy-Type Defects in Brown Diamonds Investigated by Positron Annihilation. V.Avalos, S.Dannefaer: Physica B, 2003, 340-342, 76-9