Abstract: A thermostable polycrystalline diamond body is produced by pre-treating the diamond crystals with molten base or salt, mixing the pre-treated diamond crystals with silicon powder or a blend of silicon and one or more substances, subjecting the mixture to a combination of high-pressure and high-temperature and maintaining the pressure and temperature for a period of time. The thermostable polycrystalline diamond body is characterized in having diamond crystals uniformly distributed in the body. The diamond crystals are covered by .beta.-silicon carbide and that the diamond crystals together with .beta.-silicon carbide form an uninterrupted framework throughout the entire body, the interstices of which are filled with elemental silicon or silicon-containing refractory phases. A polygonal graphite mold having walls of equal thickness useful in the production of said thermostable polycrystalline diamond body is also provided.