The Mwadui kimberlite was discovered in 1940 by Dr John T Williamson and is the largest primary deposit of it’s kind in the world ever to be mined, measuring some 146 ha at surface. This Cretaceous-aged kimberlite has been mined continuously as an open cast operation since it’s discovery. The Mwadui kimberlite can be subdivided into five major and distinct rock types, viz. the Shale Basin, the Bouma Facies, Granite Breccias, Reworked Volcaniclastic Kimberlite types (RVK), and primary Pyroclastic Kimberlite (PK).
	The Shale Basin represents a phase of lacustrine sedimentary deposition towards the end of the in-fill cycle of the crater and forms the upper part of the central depositional crater. Diamond grade is extremely low and the material has been avoided during mining. The Bouma facies represents the lower part of the central depositional crater and is distinguished from the Shale Basin by the presence of Shale Basin by the presence of Bouma sequences. Diamond grades of this material are generally low. The lower contact of the central depositional basin is characterized by a substantial Granite Breccia zone. These deposits formed after the final eruptive episode, mainly as a result of collapse of fractured and unstable crater walls, comprising basement granite. The breccia units are characterized by a large range in intermixed volcaniclastic material, resulting in material containing nearly barren to economic diamond grades. The Reworked Volcaniclastic Kimberlite (RVK) deposits were formed in response to reworking of the primary tuff ring deposits as a result of grain flow and debris/avalanche deposits into the open crater. Wall-rock dilution is relatively low compared to some of the other units, resulting in high diamond grades. The material below the RVK can collectively be referred to as Pyroclastic Kimberlite (PK) and can only be observed in the deeper sampling tunnels and drill holes. The kimberlite exploited the existing joint and fracture system during emplacement and the northern and southeastern parts of the crater terminate into hypabyssal facies dykes. The northern dyke can be classified as a macrocrystic monticellite kimberlite with variable phlogopite and microlitic diopside. The southern dyke is poorly exposed and highly weathered.