Synthetic diamond single crystals synthesized with FeCo as solvent/catalyst were heated at 1100 and 1650 degrees C for up to 1 h in vacuum. X-ray diffraction photographs taken at room temperature showed the presence of (i) body-centred cubic randomly oriented FeCo inclusions, (ii) slightly textured graphite and (iii) slightly textured compressed graphite. The FeCo inclusions remained stress-free even in crystals with the maximum, 30-35 kbar, of the observed residual graphite compression. It is concluded that the compressed graphite is located in cracks extending from the FeCo inclusions and that it is the cause for failure of synthetic diamonds on heating.