Scientists have suggested that rare black "carbonado" diamonds - found only in Brazil and the Central African Republic - may have arrived on Earth aboard a kilometre-wide asteroid, New Scientist reports.

The rare gems are, unlike other diamonds, "made of millions of diamond crystals that are stuck together", and are also porous. They are not found in conventional diamond fields and have to date been unearthed in just two locations.

Haggerty and his colleagues subjected carbonados to an infrared synchrotron at Brookhaven National Laboratory in New York and found they contained hydrogen indicating they originated in "hydrogen-rich interstellar space". The spectral tests showed they "closely resemble" diamonds found in other asteroids and those observed in space, and are between 2.6 and 3.8bn years old.

The "diamond dust" from which they eventually formed "may have been released when a star exploded in a supernova billions of years ago", NS explains. These particles subsequently "coalesced into larger clumps that became embedded in asteroids".

Regarding the geographical spread of the black rarities, the asteroid impact would have occurred billions of years ago when Africa and South America were still joined. ®

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