A method of growing single crystal diamonds in excess of 10 .mu.m in diameter from industrial diamond "seeds" having mean diameters of approximately 1.5 .mu.m is disclosed. The diamonds are grown by exposing the seed diamonds to C.sub.70 in the presence of elemental reducing agents such as phosphorus or selenium in evacuated cells at moderate temperatures and pressures. In another aspect the invention diamonds are grown by exposing diamond seed particles to vapour phase C.sub.70 in the presence of a gas phase metal carbonyl, such as F.sub.5 e(CO) in a temperature range of 400.degree. C. to 700.degree. C. to cause at least some of the diamond seed particles to grow.