The internal morphology of a natural twinned diamond was investigated using X-ray section topography. The diamond consisted of two crystals joined along a {1 1 1} plane whose remote ends were triangular {1 1 1} faces with sizes approximately 4 mm on the edge. A coating of fibrous growth obscured the morphology of the good quality diamond inside. Although the coat displayed re-entrant surfaces near the twin plane along three edges of the crystal, X-ray topography showed the inner crystal to protrude outwards along these same edges. The good quality inner core displayed the classical "spinel law" twinned octahedral morphology whereas the fibrous rim showed a typical sphalerite-like twinned tetrahedral morphology. A possible growth mechanism which could account for this is discussed. (c) 2005 Elsevier B.V. All rights reserved.