Diamonds are synthesized from SiC at temperatures and/or pressures lower than those required to convert amorphous carbon or graphite to diamond, by heating the SiC in the absence of another non-diamondaceous source of elemental carbon and in the presence of a reactant which selectively reacts with the Si at the temperature to which the SiC is heated, and in a matrix which is frangible when cooled, while the Sic is within the diamond stable region of the diamond-graphite phase diagram, thereby permitting the diamond to be separated therefrom by physical means.