PURPOSE: To prolong the service life of the gauge surface of an earth boring drill bit or rotary cutter by using an improved insert containing dispersed diamond crystals formed through the in-situ conversion of graphite into diamond for the drill bit.

CONSTITUTION: Diamond crystals formed through the in-situ conversion of graphite into diamond by using a tungsten carbide and diamond catalyst are used in an insert 41. The insert 41 is used for the formation of the member of a rotary rock bit 11. At the time of forming the diamond crystals, the grain sizes of the dispersed diamond crystals and tungsten carbide are adjusted to about 0.2-5 μ and W, Ti, Zr, Hf, V, Nb, Ta, Cr, or Mo powder is used as metal carbide powder. In addition, a group III-VI metal or Ni, Co, Fe, Al, or B is used as the catalyst and the grain size of the graphite is adjusted to about 0.2-1,000 μ. Moreover, the mixing ratio of a binder in a particle compound is adjusted to about 5-50 vol.%. Furthermore, the sintering condition of a green material is set at lower than about 1,500°C in temperature and lower than about 1,000 psi in pressure. A sintered material is treated in an HPHT device maintained at about 1,200°C or higher in temperature and about 40 kilobar or higher in pressure.