Combined TEM and STEM study of the brown colouration of natural diamonds

A range of natural brown, colourless and HPHT-treated single crystal diamonds are analysed via Transmission Electron Microscopy (TEM). Graphitic inclusions are found only in specimens originating from diamonds with brown colour. The inclusion sizes range 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 { 110} planes respectively, where three diamond {111} planes coincide with two graphitic basal planes. This orientation, suggesting diamond 111-vacancy platelets, is consistent with defects associated with a featureless optical absorption spectrum similar to brown diamond. Vacancies and dislocations in diamond and their link to brown colouration will continue to be studied.