High pressure sintered bodies of polycrystalline diamond or carbide ceramics are rendered more resistant to cracking by establishing dispersed, localized "stress centers" of submicron size particles of lonsdaleite, diamond, or other form of metastable, high density carbon which has the capability of transforming crystallographically to graphite in situ in the body. When an incipient crack in the body approaches entrapped metastable particles along its path, the crack promotes or permits transformation of the metastable particles to graphite particles, with concomitant increase in their size, thereby resisting further propagation of the crack.