An experimental study on diamond nucleation and growth in a carbon solution in multicomponent carbonate, carbonate-silicate, silicate and sulfide melts was performed using a high-pressure toroidal anvil-with-hole cell with graphite resistive furnace. The boundary conditions of diamond spontaneous crystallization and seeded growth are determined for all the diamond growth media with the use of the PT diagram of diamond crystal growth. A density of diamond nucleation in the studied carbonate, carbonate-silicate, silicate, and sulfide melts with dissolved carbon exceeds (3.0–5.0) × 102 nuclei/mm3 with a maximum around 1.0 × 105 nuclei/mm3 at formation of polycrystalline “diamondite”. Raman spectra of quenched carbonate-carbon, carbonate-silicate-carbon, silicate-carbon and sulfide-carbon melts contain bands relating to the region of C-C stretching modes in diamond and graphite microphases. FTIR spectra show that nitrogen defects in the carbonate-synthetic diamond are characterized with the mixed Ib-IaA type and reveal a high nitrogen concentration (up to 850 ppm).