It is demonstrated that the individual grains of polycrystalline, chemical vapour-deposited (CVD) diamond are arranged in clusters composed of several tens of individual grains and related by twinning. Transmission electron microscopy (TEM) analysis showed that for (110)-oriented growth these clusters become very large but the size of the individual twins within them are relatively small. Geometrical effects within a grain cluster can lead to large internal stresses. Scanning electron microscopy (SEM) methods are described to distinguish grain clusters from the individual grains within them. (C) 2003 Elsevier Science B.V. All rights reserved.