Abstract: Cubic boron nitride crystals are grown by subjecting reaction materials of low pressure phase boron nitride, a solvent material, and cubic boron nitride seeds to pressure and temperature conditions in the cubic boron nitride-stable region. The reaction materials are in the form of a pair of a superimposed solvent material plate and a low pressure phase boron nitride plate or a pile made of a plurality of the pairs of the superimposed solvent material plate and low pressure phase boron nitride plate, and a plurality of the seeds are disposed on either one or each of the confronting surfaces of the pair of the superimposed solvent material plate and low pressure phase boron nitride plate. Alternatively, the reaction materials are in the form of a plate made of a mixture of the solvent material and the low pressure phase boron nitride or a pile made of a plurality of the mixture plates, and a plurality of the seeds are disposed on a surface of each plate. The seeds have a particle size of not larger than 150 .

Abstract: Diamond crystals are grown by subjecting reaction materials of nondiamond carbon, a solvent metal, and diamond seeds to pressure and temperature conditions in the diamond-stable region. The reaction materials are in the form of a pair of a superimposed solvent metal plate and a nondiamond carbon plate or a pile made of a plurality of the pairs of the superimposed solvent metal plate and nondiamond carbon plate, and a plurality of the seeds are disposed on either one or each of the confronting surfaces of the pair of the superimposed solvent metal plate and nondiamond carbon plate. Alternatively, the reaction materials are in the form of a plate made of a mixture of the solvent metal and the nondiamond carbon or a pile made of a plurality of the mixture plates, and a plurality of the seeds are disposed on a surface of each plate. The seeds have a particle size of not larger than 50 .mu.