Experimental study of diamond nucleation and growth in pyrrhotite-carbon melts was carried out at 6.0–7.1 GPa. The PT -diagram of diamond crystallization was constructed, and we found that the PT boundary conditions of diamond formation are limited by the diamond-graphite equilibrium line and the PT curve of the pyrrhotite-carbon eutectics. The fields of diamond spontaneous nucleation and mass crystallization as well as of diamond-seeded growth were determined. Diamond nucleation density up to 1.8 × 105 grains in 1 mm3 and growth rate within the range of several mm/min to several μm/min were estimated. A possibility for formation of both monocrystalline and polycrystalline diamonds in the molten sulfide solvents was also demonstrated. Experimental data are in agreement with the melt-solution model of diamond formation in sulfide solvents of carbon. The experimental and literature data applied to the model of the mantle-derived diamond genesis demonstrate that sulfide melts with dissolved carbon are capable to form a limited mass of diamonds with specific mineralogical and physical properties.