The results of a study of diamond crystallisation in dry and fluid-rich dolomite-carbon systems are presented. For the dry system an induction period preceding spontaneous diamond nucleation was found of about 4 h at 7 GPa, 1700 °C. No diamonds were observed after 42 h of reaction at 5.7 GPa, 1420 °C. Adding H2O and H2O-CO2 fluids to the dolomite-carbon system resulted in spontaneous diamond nucleation at 1420 °C, and growth of diamond on seed crystals at 1300–1420 °C. In the presence of H2O or H2O-CO2 fluids, dolomite decomposes to dolomite solid solution + brucite + aragonite. Results of the experiments indicate that dolomitic melts in the mantle, enriched in H2O and CO2, promote the formation of natural diamond.