The evolution of etch phenomena on (111) natural octahedron faces of a number of good-quality diamonds, produced by immersion in hot potassium nitrate in temperatures ranging from 500 to 700 degrees C, is studied optically. The techniques employed are high-resolution microscopy under a variety of illumination conditions, multiple-beam interferometry and light-profile microscopy. Although the etching mechanism is continuous it is conveniently divided into three stages. It is established that the first stage begins with a preferential attack on any existing surface flaws and then develops into a random distribution of a large number of small etch-pits. The distribution of these is random, but the concentration of etch-pits is less within any growth trigons than elsewhere, and it is presumed that this is because of a lower concentration of dislocations within the trigons. In the second stage, a relatively small number of etch-pits grows steadily and these devour their smaller neighbours. The corners become rounded and flat-bottomed pits are frequent. An explanation is proposed. In this stage any growth trigons tend to become hexagonal in outline, and the mechanism for this is unfolded and described. The third stage occurs when the whole of the original face has been eaten away. It is characterized by spread and deepening of the pits such that at first they become triangular pyramid depressions and then ultimately a striking block formation results. It is established by optical measurement that the pile of blocks have plane sides which are mainly (221) (212) (122) faces and as etch develops less prominent (331) (313) (133) faces develop. Ultimately, these tend to become (334), etc., but then rounding sets in and the crystallographic faces tend to lose their character.