Growth habit of needle crystals in coats of coated diamonds

Abstract

The growth habit of needle-shaped crystals in coats of coated diamonds from Zaire was observed using a cathodoluminescence microscope. An aggregate of needle crystals of diamonds coats a single crystal of core in an octahedral shape, and the tips of the needle crystals in the coats are bounded by curved {100} faces (or cuboids) with small {111} facets, showing the distinct sector-zoning in CL brightness. The needle is responsible for ‘random walk model’, which can explain the external morphology of coated stones. This characteristic growth of the needle crystals were interpreted to be formed by the co-precipitation of inclusions and due to the rapid growth controlled by the kinetic roughening of the octahedral faces and the kinetic smoothening of the cuboid faces. Morphology of the needles to those of diamonds by mixed-habit growth suggests that both types of diamonds have grown under similar conditions, except for the co-precipitation of inclusions.