Abstract: A method for bonding a cubic boron nitride sintered compact to other cubic boron nitride sintered compact or to a body of shank material is disclosed. The method comprises forming a Ti layer of 0.01-1 .mu.m in thickness over a bonding interface between two cubic boron nitride sintered compacts or between a cubic boron nitride sintered compact and a body of shank material, forming a layer of Ni or Cu over the Ti layer to a thickness of 0.01-5 .mu.n, putting together the two cubic boron nitride sintered compacts or the cubic boron nitride sintered compact and the body of shank material with a 10-1,000 .mu.m foil of Al, Al-Ni alloy or Ag--Cu--In alloy being placed over the boding interface, and heating the cubic boron nitride sintered compact structure to temperatures above the meeting point of the metal foil and not exceeding 750.degree. C. in an inert atmosphere or in a vacuum.

Abstract: A process for synthesizing a large diamond having a diameter of 8 mm or more by the temperature gradient method, wherein a (111) or (100) surface of a seed crystal having a diameter of 3 mm or more is used as a growing surface, the entire area of the growing surface is first dissolved in the diamond-stable region before crystal growth is started, the crystal growth is effected using a plug of a solvent in which the height of the central portion thereof is higher than the height of the peripheral portion thereof, the plug of a solvent has a planar or curved surface on the side where the plug of a solvent contacts a carbon source during the crystal growth, and the crystal growth is effected under such pressure and temperature conditions that the growth of the (111) or (100) surface is predominant.