An ultrasmooth-diamond film has a thickness greater than about ten microns and an average grain size less than about 0.5 micron. The ultrasmooth diamond film of the present invention is grown using ordinary microwave plasma CVD methods, with a metal vapor source included in the reactor to produce vapor during the growth of the film. The metal vapor source may be chosen from the first row transition elements, chromium, iron, cobalt, and nickel, or from the lanthanides praseodymium, europeum, or erbium. Any metal capable of existing in the vapor phase in the presence of the hydrogen plasma, will cause formation of the ultrasmooth film of the present invention.