Linear decorations defining edges of an internal octahedron within a natural diamond: observations and an explanation

Abstract

A 2.13 carat weight natural diamond, a slightly flattened octahedron with roughly-pitted surfaces, exhibits an internal framework of dense black “lines” located a short distance below the present surface. Most of the lines lie closely parallel to 〈110〉 and connect up to delineate edges of an octahedron concentric with and slightly smaller than the present body. The lines are agglomerations of thin black discs (thickness too small to be measured in situ by optical microscopy), diameters ranging from 1 to 40 μm (but mainly in the 5 to 20 μm range), equantly distributed on diamond {111}, and consistent with graphite. Special optical techniques were developed for examining this specimen and they provided control of relative visibility of internal versus superficial features. Optical micrographs taken in all eight 〈111〉 directions are discussed. Synchroton X-ray topography detected the internal linear framework with good contrast, and helpfully displayed spatial relations between its segments. Findings from cathodoluminescence topography are described. An explanation of the phenomenon is outlined. It proposes a sequence of pressure and temperature conditions and of resulting stresses between diamond and encasing solid matrix whereby graphitization was caused to occur locally along edges of an octahedron, which was subsequently enclosed by further diamond growth. Implications for the origin of the edge grooves frequently exhibited by diamond octahedra are suggested.