Diamond heated in vacuo to about 2000° C. is wholly or partially converted to graphite which has a strong preferred orientation, the [0001] axis of the graphite being associated with the diamond [111] axis irrespective of the original habit of the diamond. The preferred orientation of the graphite in cliftonite, however, has been shown to involve the association of the [0001] axis with the cube axes of the crystal, thus casting doubt on the possibility that cliftonite is a pseudomorph after diamond. The graphite itself appears in both cases to be imperfectly crystallized, and is neither α- nor β-graphite, nor the usual mixture of the two. It has been shown by means of divergent-beam X-ray photographs that the Inosaicity of diamonds can be increased by heat-treatment.