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Carbonado diamonds are typically pea-sized or larger porous aggregates of many tiny black crystals. The most characteristic carbonados have been found only in the Central African Republic and in Brazil, in neither place associated with kimberlite, the source of typical gem diamonds. Lead isotope analyses have been interpreted as documenting crystallization of carbonados about 3 billion years ago. The carbonados are found in younger sedimentary rocks.
Mineral grains included within diamonds have been studied extensively for clues to diamond origin. Some typical diamonds contain inclusions of common mantle minerals such as pyrope and forsterite, but such mantle minerals have not been observed in carbonado...
Extraterrestrial origin hypothesis
Supporters of an extraterrestrial origin of carbonados, such as Dr. Stephen Haggerty, a geoscientist from Florida International University, propose that their material source was a supernova which occurred at least 3.8 billion years ago. After coalescing and drifting through outer space for about one and a half billion years, a large mass fell to earth as a meteor approximately 2.3 billion years ago, possibly fragmenting during entry into the Earth's atmosphere, and impacting in a region which would much later split into Brazil and the Central African Republic, the only two known locations of carbonado deposits.