Diamond crystallization in the Mg-Si-C system has been studied at high-pressure higherature conditions of 7 GPa and 1500-1900 °C. The features of nucleation and growth of diamond from the carbon solution in the Mg-Si melt are established. The degree of the graphite-to-diamond transformation is found to depend significantly on the crystallization temperature. As opposed to the pure Mg-C system where the cubic morphology dominates, the octahedron with the antiskeletal structure of faces is the dominant form of growth in the Mg-Si-C system over the entire temperature range. The possibility of epitaxial growth of silicon carbide tetrahedral crystals on diamond upon their co-crystallization was noted. Synthesized diamonds are found to contain optically active silicon-vacancy (Si-V) centers and inactive substitutional silicon defects, giving rise to the 1.68 eV system in the photoluminescence spectra and an absorption peak at 1338 cm-1 in the infrared absorption spectra, respectively.