Growth morphology appearing as external forms and zonal structures of spinel-twinned crystals of natural diamond from South Africa was studied. The crystals are divided into four groups depending upon the modification of their re-entrant corners by growth layers generated at the twin boundary. The flattened morphology is found to be caused by the preferential growth at both re-entrant and salient corners, the latter being revealed for the first time by this paper. The change of supersaturation conditions was also estimated from the morphological change. (C) 2002 Elsevier Science B.V. All rights reserved.