A range of natural brown, colorless and high-pressure high-temperature treated single crystal diamonds were analyzed by using transmission electron microscopy. Graphitic inclusions were found only in specimens originating from diamonds with a brown color. The inclusion sizes ranged from slivers of around 50nm by 5-10nm, to regions around 10μm in diameter with undefined boundaries. The suspected proportions of graphite to diamond were confirmed by micro-Raman spectroscopy. The orientation relationship, measured via diffraction and lattice imaging, was found to be diamond {111} and {110} planes parallel to graphitic basal and {¯21▪0} planes respectively, where 3 diamond {111} planes coincide with 2 graphitic basal planes. This orientation, suggesting diamond 111-vacancy platelets, was consistent with defects associated with a featureless optical absorption spectrum similar to brown diamond.

Combined TEM and STEM Study of the Brown Colouration of Natural Diamonds. R.Barnes, U.Bangert, P.Martineau, D.Fisher, R.Jones, L.Hounsome: Journal of Physics - Conference Series, 2006, 26, 157-60