The external forms and zonal structures of spinel-twinned crystals of natural diamond were studied. The crystals were divided into 4 groups, depending upon the modification of their re-entrant corners by growth layers generated at the twin boundary. A flattened morphology was found to be caused by preferential growth at both re-entrant and salient corners; the latter being revealed here for the first time.

Growth Morphology and Change in Growth Conditions of a Spinel-Twinned Natural Diamond. A.Abduriyim, M.Kitamura: Journal of Crystal Growth, 2002, 237-239[2], 1286-90