broadly , the present invention is directed to a high pressure / high temperature process for making polycrystalline cbn composite masses from graphitic boron nitride in the substantial absence of catalytically - active material . the present invention specifically comprises subjecting a mixture of different structural forms of gbn to simultaneous conversion of each for making a cbn / cbn composite mass . pyrolytic boron nitride / hexagonal boron nitride mixtures are ideally suited for conversion into the inventive cbn / cbn composite masses of the present invention .