Carbonado diamonds, sometimes known simply as “black diamonds” are enigmatic things. Given their origins, rare and expensive seems something of an understatement. Regular diamonds (allegedly a girl’s best friend) are created naturally inside planet Earth, through the high pressures found deep in the Earth’s crust. They’re found and mined in all manner of places across the world. Carbonados, on the other hand, are only found in two places on the globe. The Central African Republic and Brazil. There’s actually a very good reason why carbonados aren’t strewn across the world like most other diamonds. They’re not from this world.

Carbonados are black, because they’re polycrystalline. In fact, they’re cryptocrystalline — composed of crystals so tiny, they can be smaller than the wavelengths of light. They’re difficult to see, even with a microscope. At the same time, they’re all bubbly and spongey-looking. They look more like pumice stones than gemstones.

The reason why carbonados are so weird ties in with how they’re created. Some scientists are convinced that they’re formed in supernovae. Specifically, a star exploded some billions of years ago. When it did, it sent huge chunks of starstuff hurtling into the night, cooling and solidifying into a slew of different supernova condensates. Fragments of superheated carbon from the star’s interior would also have been flung out. Roiling, frothing globules, cooling from insane tempertures, their exteriors trying to crystallise as they rapidly lost their heat.

Fast forward a few million years, and asteroid size chunks of black diamond are still speeding though space. Inevitably, some get captured in the gravitational fields of stars. One such piece of supernova debris, it’s believed, collided with Earth millions of years ago. What wasn’t vapourised on contact, fragmented into pieces, peppering the ground like a shotgun blast. Waiting for humans to discover them in river sediments. At the time, Africa and South America were still connected. In the following ages the continents drifted, giving the two places where carbonados are now found on Earth.

(Actually, experts believe the carbonado fragments were originally embedded inside a larger piece of space rock which has since been worn away… but I’m feeling melodramatic. Plus both mechanisms would still give the tiny rounded diamond fragments found in the riverbeds.)

All of this might sound a little far-fetched, but there’s a lot of supporting evidence. Spectroscopically, they certainly look more like what you might find in interstellar space. Their isotope ratios are unusual, and unlike terrestrial diamonds. They’re full of hydrogen, and they contain inclusions of silicon carbide and iron-nickel clusters, as found in meteorites and stardust. What’s more, lead isotope analyses suggest they could be around 3 billion years old. That’s comparable with the age of planet Earth itself.

The supernova idea is still a hypothesis for now. The best way to prove the theory? Finding the same carbonados elsewhere in the solar system would be almost incontestible proof. Maybe there are still more chunks of carbonado languishing in the asteroid belt waiting for us to find them. If there are, it’s a safe bet we might find them somewhere around Sirius, Alpha Centauri and other neighbouring stars too.

Hmmm… I think I’ll stave off the temptation to make a pun about diamonds being “forever”.

Image credit: Steve Haggerty.

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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