A method of producing diamonds comprises the steps of producing a carbonaceous powder comprising nano-structured carbonaceous material and a transition metal and thermally treating the powder. The carbonaceous powder is produced by electrochemical erosion of graphite in a molten salt, the transition metal being incorporated into the carbonaceous powder during the electrochemical erosion. The step of thermally treating the carbonaceous powder is carried out in a non-oxidising atmosphere at a temperature of between 350° C. and 300° C., at a pressure of lower than 1 GPa. The method allows diamond to be produced at low pressures and low temperatures.