Pressure crack-figures on diamond faces II. The dodecahedral and cubic faces

Abstract

The study of pressure figures on diamond is extended to the cubic and dodecahedral faces. The formation of cracks on a sawn but unpolished dodecahedral (two-point) face is first studied. The loads required for the initiation of cracks, using a diamond ball, were between 7 and 24 Kg. Two different types of cubic face were available, one on a natural cubic boart stone, the other a polished approximation to a cube face secured by truncating an octahedral stone. The loads required to initiate cracks were of the order 20 to 30 Kg. All the cracks observed were specifically oriented in accordance with crystallographic expectations of easy cleavage directions. The cracks were accompanied by permanent surface distortions which were studied by multiple-beam interference methods. The permanent distortions broadly resemble those found in part I for octahedral faces. There is one important difference in that for both the cube faces studied, the surface level within the perimeter of the ring crack is appreciably depressed, being some 800 Å below the outer undisturbed level. This is considered to offer further evidence for the existence of plastic flow in diamond at room temperature. For the polished cube face studied observations could also be made on the accompanying internal cracking effects within the body of the crystal.