Carbonado diamondites (small specimen at top: 4.05 carats and 9x9x6 mm in size; medium-sized specimen at left: 4.07 carats and 10x13x7 mm in size; large specimen at right: 5.09 carats and 12x6x6 mm in size).
Diamondites are rare, polycrystalline, nearly monomineralic diamond rocks. The term “diamond” refers to a mineral, while the term “diamondite” refers to a diamond rock. The three diamondites depicted here are a variety referred to as “carbonados” in the diamond trade. Carbonados are typically a dark brown to blackish color. The shiny luster seen in the specimens below is typical for carbonados (adamantine luster). Individual diamond crystals in carbonado diamondites are exceedingly small, typically <20 m. Carbonados are also characterized by having obvious porosity in all portions of the rock (see pits in specimens shown above).
Despite a relative abundance of published research, the origin of carbonado diamondites is still unresolved. Some workers interpret them as being crustal in origin. Some consider them to have a mantle origin. Some suggest that they are impact-generated. Recent research has indicated that carbonado diamondites are extraterrestrial - extrasolar, in fact (apparently supernova-generated; see story produced by the American Institute of Physics).
True carbonado diamondites have only been recorded from Brazil and the Central African Republic (CAR). Considering South America & Africa were physically connected for a long time while a part of the ancient Gondwana continent, these two carbonado occurrences may ultimately represent one deposit. The specimens shown above are from the CAR.
In the CAR, carbonado diamondites are recovered from modern to subrecent alluvial gravels (placer deposits) below the topsoil. They are only found in & immediately adjacent to the outcrop belt of the Carnot Formation (Cretaceous). The Carnot Formation is an ancient fluvial & lacustrine siliciclastic unit that hosts scattered diamonds & carbonados. Upon weathering & erosion, any included diamonds in the Carnot Formation enter modern river systems’ sediment loads.
Age: carbonado diamondites are exceedingly old. Published research indicates they are 3.2 billion years old (mid-Mesoarchean).
Locality: unrecorded locality in the “Bangui region” (area between the cities of Bangui and Berberati), southwestern Central African Republic, central Africa.