A study of the boron-carbon-nitrogen system was carried out to synthesize new materials and examine phase relations. Crystalline graphite- and diamond-like BCN phases were synthesized by thermobaric treatment of mixtures of boron with carbon nitride or melamine powders. These BCN phases crystallized in the basic lattice of boron nitride with statistically distributed carbon atoms. New structural modifications were discovered by studying the kinetics of phase formation. The hexagonal lattice of a graphite-like phase is transformed by a diffusionless mechanism into a cubic structure at pressures of around 6.5 GPa. A diamond-like BCN phase was synthesized in the form of individual crystals or polycrystalline aggregates. The microstructure of these aggregates is comparable to natural and synthetic carbonado-type diamond. The obtained results argue in favour of the hypothesis that natural carbonados formed in the Earth’s mantle.