Twinning in Natural Diamond. II. Interpenetrant Cubes

Single crystals of natural diamond which exhibit an approximately cubic morphology are known to have grown either (a) by nonfaceted cuboid growth or (b) by fibrous growth branching in a multiplicity of <111> columns, to give a mean surface orientation of {100}. Interpenetrant twins of diamond are uncommon; but when they occur they are usually coloured and appear (by X-ray topography) to have been formed by fibrous growth upon a twin origin, with the 14 growth sectors adequately filled by fibres, without the necessity of branching.