Vacancy-type defects in brown diamonds investigated by positron annihilation

Five brown natural type IIa diamonds were found to contain concentrations of trapped monovacancy-type defects and large vacancy clusters that significantly exceed the vacancy content in colourless natural or synthetic type IIa diamonds. A proprietary heat-treatment reduced the vacancy cluster content by about 50% and removed the brown colour. The origin for the brown colour is associated with highly distorted regions.