BLACK DIAMONDS FROM MARANGE (ZIMBABWE): A RESULT OF NATURAL IRRADIATION AND GRAPHITE INCLUSIONS

This study investigates the color origin of 40 natural Fancy Dark brown-black round brilliant diamonds from the Marange alluvial deposits in eastern Zimbabwe. Visual observations show that the dark appearance of the Marange diamonds is due to a combination of graphite micro-inclusions (associated with methane), graphite needles, and dark brown radiation stains that occur along internal fractures. The GR1 (V-0) defect, typically formed during natural and artificial irradiation, is observed in the optical spectra of 43% of the diamonds, although its intensity is too low to significantly impact the bodycolor. Natural irradiation in these diamonds is likely related to their billion-year residence in the Umkondo conglomerate, which is known to contain radioactive minerals such as zircon. Aside from radiation staining, irradiation-damaged diamond appears non-luminescent in DiamondView images and shows a weaker, broader diamond peak (at 1332 cm(-1)) in Raman spectroscopy. Brown coloration of the radiation stains is due to heating of the diamonds during later regional metamorphism, which also facilitated the formation of the H3 (NVN0) and NiN complexes.