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Carbonados are an amorphous form of crystallised diamond carbon. Generally found as pea sized (or larger) pebbles, to date they have only been found in the Central African Republic and Brazil, usually embedded in sedimentary rocks.
Their origin is as yet unexplained - their main feature is their porous nature, being pumice-like. Since (true) diamonds are normally formed under extreme geological pressure, it's not possible to explain how carbonados include voids - around 10% of their structure is open.
They may have formed in deep space and arrived on Earth as an asteroid - but the means of formation is still unexplained, and stress laminations (which would presumably have formed on impact) aren't present.
See Strange Diamonds: The Mysterious Origins of Carbonado and Framesite(paywalled) Mineralogical Society of America, 2005.
Further info. Wikepedia article
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