Black Diamonds from Marange (Zimbabwe): A Result of Natural Irradiation and Graphite Inclusions

Abstract

have either natural or treated color origin. Natural black diamonds are usually colored by inclusions of sulfides, graphite, magnetite, hematite, or iron-bearing inclusions (e.g., Titkov et al., 2003). A rare natural diamond (of undisclosed geographic origin) colored by abundant brown radiation stains has previously been examined by GIA’s Carlsbad laboratory (Ardon, 2013). Treated black diamonds are often those that are heavily fractured naturally and then treated at low-pressure and hightemperature (LPHT) conditions to graphitize the fractures and turn them black (Hall and Moses, 2001; Notari, 2002). Artificial irradiation can also produce dark colors that appear black (Collins, 1982; Kitawaki, 2007). The Marange locality in eastern Zimbabwe is well known for producing diamonds that contain both octahedral and cuboid sectors (mixed-habit diamonds) where the cuboid sectors are visible to the eye due to abundant micro-inclusions of graphite (Rakovan et al., 2014; Smit et al., 2016). These micro-inclusions, informally known in the gem trade as “clouds,” give the diamonds a brown-gray appearance that lowers their value. Heat treatment of these lower-quality graphitecontaining Marange diamonds has the potential to introduce gem-quality treated black diamonds into the market. In natural diamonds, these graphite micro-inclusions are around 1 μm in diameter; during heating above 1200°C, they become larger. After annealing at 1700°C, the grain size increases to 11–16 μm, causing the cuboid sectors to appear opaque black (Eaton-Magaña et al., 2017). The challenge for gem laboratories is to confidently distinguish these treated black diamonds from naturally occurring black diamonds. Here our goal was to document a suite of untreated Marange diamonds, all with Fancy Dark brown to Fancy black GIA color grades, so that their characteristics could be distinguished from any suspected treated black diamonds. When viewing the samples, however, it became clear that the appearance of the these dark Marange diamonds was due not only to graphite clouds but also to abundant graphite needles and dark brown radiation stains occurring within surface-reaching fractures. BLACK DIAMONDS FROM MARANGE (ZIMBABWE): A RESULT OF NATURAL IRRADIATION AND GRAPHITE INCLUSIONS