We report for the first time experimental evidence for the nucleation and growth of diamonds from carbonatitic melts by reduction in reactions with silicon metal or silicon carbide. Experiments were carried out in the CaMg(CO3)2-Si and CaMg(CO3)2-SiC systems at 7.7 GPa and temperatures of 1500–1800 °C. No graphite was added to the run powder as a carbon source; the carbonate-bearing melts supply the carbon for diamond formation. Diamond grows spontaneously from the carbonatitic melt by reducing reactions: CaMg(CO3)2 + 2Si = CaMgSi2O6 + 2C in the CaMg(CO3)2-Si system, and CaMg(CO3)2 + 2SiC = CaMgSi2O6 + 4C in the CaMg(CO3)2-SiC system. Our results provide strong experimental support for the view that some natural diamonds crystallized from carbonatitic melts by metasomatic reducing reactions with mantle solid phases.