Pumicelike, dark, and exotic, carbonado diamonds don’t look like the gemstones on engagement rings. That’s because they may have been created by an exploding star and delivered to Earth by an asteroid billions of years ago.
From Canada to Australia, most diamonds were formed at torturously high temperatures and pressures deep within Earth before being carried to the surface by explosive volcanoes. But the frothy texture of carbonados—found only in Brazil and the Central African Republic—is too full of bubbles to cut and polish into a gem. They are “just totally incompatible with a deep-earth origin,” says Stephen Haggerty, a geologist at Florida International University.
Haggerty and his colleagues analyzed the chemical composition of the diamonds by bouncing infrared light off polished slivers. The resulting spectra, reported in The Astrophysical Journal Letters in December 2006, don’t match signatures for terrestrial hydrogen and nitrogen but instead mimic those found in interstellar space. The findings, Haggerty says, boosted the theory of carbonados’ extraterrestrial origin from 4 to 9 out of 10 on the credibility scale.
The age of these diamonds is between 2.6 billion and 3.8 billion years, dating to a time when South America and Africa were joined. No carbonados have been found anywhere else on Earth, leading Haggerty to believe they all originated from a single asteroid impact. But he doesn’t think that one giant carbonado fell from the sky. All the stones are opaque and porous, but their colors vary from black and gray to green and even red, which tells Haggerty that it is more likely they arrived here embedded within another rock. The matrix that held them has now weathered away, leaving a scattering of stones.
What would confirm the stones’ origin once and for all? Finding an object with similar diamond spectra in the asteroid belt, Haggerty says. “That would make it a 10."
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