Black material on mineral inclusions and in internal fracture planes in diamond

The black material observed on the surface of crystalline inclusions in diamond and also in adjacent internal fracture planes has been classified as a result of a preliminary examination of several hundred diamonds from Sierra Leone, Ghana, and South Africa (particularly the Premier Mine), as well as diamond slices of unknown origin, followed by a detailed examination of some 100 diamonds. X-ray, electron diffraction, and qualitative electron microprobe techniques were used in identifying this material as graphite, pyrrhotite and pentlandite. The possible origins of these minerals are discussed.