A process and apparatus for the synthesis of diamond from carbon precursors in supercritical water is described. The method involves the oxidation and/or pyrolysis of gaseous, liquid or solid phase carbon precursors in supercritical water (described by a critical pressure and critical temperature of at least 218 atm and at least 374.degree. C. respectively. The oxidation and/or pyrolysis products approach equilibrium concentrations in a reactor maintained at supercritical conditions for the mixture of supercritical water (solvent) and added reactants (solute). Besides carbon containing reactant precursors, additional reactants such as oxygen or hydrogen serve to enhance the oxidation/pyrolysis process. The reaction products undergo retrograde condensation resulting from imposed temperature gradients. The condensate is solid carbon, in the chemical form of carbon that is sp.sup.3 bond coordinated and of cubic or wurzite symmetry. The condensation process is enhanced by the presence of a seed diamond crystal.