The carbonado diamond conundrum

The reviewer presents the results of a literature search on the enigmatic occurrences of carbonado; a form (but not the only form) of polycrystalline diamond, which is mined for industrial diamonds. The principal occurrences are in Brazil, the Central African Republic (CAR) and Siberia (Yakutia). In the latter the name yakutite has been coined, but, though there are differences between these polycrystalline diamonds and the Brazil/CAR minerals, they would seem to be little more significant than differences within the ‘Gondwana’ occurrences, and ‘yakutite’ should probably be regarded as another name for carbonado. Most occurrences are in placers and the minerals have probably been recycled more than once by sedimentary processes, the ultimate source being unknown. Polycrystalline diamond has been described from the Udachnaya pipe, Yakutia and, though it is strictly framesite (defined as an eruptive pipe variety of carbonado), it differs little from other carbonados. The fact that all occurrences known are in areas with kimberlites and gem diamond developments present, suggests that carbonado does stem from terrestrial eruptive processes, though not necessarily kimberlite pipes. Nevertheless, this can be only a provisional, personal conclusion, there is much contradictory evidence in the literature. The evidence of subjection to radiogenic agencies in CAR carbonados seems to be unexplained, but it may be a secondary effect and not present in all carbonados. This is a conundrum which still calls for much more research.