Abstract: A colorless and transparent, substantially inclusion-free diamond crystal which can be applied to decorative uses and optical parts is synthesized by a process using a temperature gradient method in an ultra-high pressure apparatus. This process comprises using, as a solvent for the growth of the crystal, at least one metal selected from the group consisting of Fe, Co, Ni, Mn and Cr (at least two metals in the case of containing Fe) and as a nitrogen getter for the removal of nitrogen in the solvent, at least one metal selected from the group consisting of Al, Ti, Zr, Hf, V, Nb and Ta in a proportion of 0.5 to 7% by weight (at most 2% by weight when using only Al) to the solvent metal.