Catalytic etching of synthetic diamond crystallites using small cobalt particles in a hydrogen atmosphere was found to be dependent on the crystal planes of diamond crystallites. The etched surface formed on the {100} planes at 900 degrees C consisted of reversed pyramid-like etch pits with {111} walls. Those on the {111} planes conversely showed a characteristic morphology consisting of nanochannels with flat walls perpendicular to the flat {111} base.