Positron annihilation and optical studies of natural brown type I diamonds

•Positron annihilation measurements of natural brown type I diamonds were performed.•Natural brown type I diamonds were found to contain large vacancy clusters.•Vacancy cluster defects were absent in clear diamonds.•The concentration of vacancy clusters increased with brown colouration intensity.•These results indicate that large vacancy clusters are the brown centres.

Positron annihilation measurements of natural brown type I diamonds revealed that they contain monovacancy-type defects and large vacancy clusters. The latter defect type was absent in clear diamonds. Both the intensity of the positron vacancy cluster lifetime and the S-parameter were found to increase with the intensity of brown colouration determined by optical absorption measurements. No correlation was found between the nitrogen content and the positron annihilation parameters.