Optical methods to select diamonds of very high lattice perfection

Abstract

WE describe here recent developments which permit the selection of diamonds very free from strain by purely optical inspection. Figure 1a shows the edge of a diamond slab which chipped while being polished, and Fig. 1b the same slab viewed between crossed polars¹. The birefringent pattern indicates a region of strain centred near the crack, and a dark area where the chip has come away relieving the strain. This type of strain birefringence is, of course, well known, and by carefully selecting diamonds on the basis of their birefringence it is possible to obtain stones for tools with a superior resistance to chipping. For example, the six similar tools in a wear experiment described by Casey and Wilks² were chosen in this way, and wore steadily without any chipping. Bell, Wilks and Wilks³ have described how the resistance of a diamond to cracking and chipping may be estimated by measuring the load on a small diamond sphere required to produce a ring crack on a flat diamond surface. In these experiments, measurements on a diamond slab chosen to be free of birefringence gave consistent values of the fracture load which were about twice as high as those on another slab which showed marked birefringence.