Steep-sided trigons on diamonds

The natural octahedron surfaces of diamond exhibit many small triangular-shaped cavities or trigons. The majority are low-angle pyramids, but there is another variety which has steep sides and a flat base. Contour measurements on the pyramidal form have already been reported; similar measurements have now been made on the steep-sided variety. It is found that the sides of these trigons are octahedron planes. Thus the steep-sided trigons must arise by some quite different mechanism from the etch process responsible for the low-angle pyramids.