Natural brown (the most common colour) and colourless type-IIa diamonds containing only trace quantities (< 1at.ppm) of nitrogen were compared. Numerous attempts were made to trace the origin of brown tints in natural diamond, with the most likely culprits, i.e. dislocations and nitrogen impurities, ruled out through the application of various analytical techniques. Consequently more emphasis had recently been placed on the study of smaller defects in the diamond structure and their influence on colour. The focus of this research work was the analysis of vacancy defects having a size of the order of 1nm using aberration-corrected scanning transmission electron microscopy. The small electron probe size and depth of focus afforded by this technique allowed such defect structures together with their position to be resolved far more readily than with conventional high-resolution transmission electron microscopy. Small-scale contrast variations were apparent in the lattice images of brown and not of colourless diamonds. These features were compared to simulated phase contrast images of vacancy clusters in diamond. In addition, both experimental and simulated defocus series indicated that such features were not restricted to the surface of the specimen.

Atomic Structure-Colour Relationship in Natural Diamonds. I.S.Godfrey, U.Bangert: Journal of Physics - Conference Series, 2010, 241[1], 012053