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There are still lots of unsolved mystery about carbonados ("black diamonds"), and geologists even think those mystery rocks come from outside of Earth.

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Carbonados, sometimes referred to as black diamonds, have been on geologists' radar since their discovery in 1841. They look neat, and have some cool industrial uses.

But there's a lot we don't understand about this particular flavor of sparkly rock. Like where on Earth they come from. ... If they come from Earth at all.

As you might expect, carbonados are black. Or at least, they're dark. They're opaque, and really can range anywhere from grey to pink to green.

And they're sometimes used in jewelry. But because carbonados are rarer than diamonds, much of the black diamond jewelry on the market is made from naturally occurring diamonds with darker colors, or ones that have been treated to change their color. On the other hand, carbonados, though made of diamond, are a little different from the one you might be picturing.

They're crystallized carbon, so they still fall into the diamond family, but instead of tight, continuous, uniform crystals like most diamonds, they're aggregates of a bunch of randomly arranged microcrystals. This arrangement makes carbonados something geologists refer to as a xenomorph. In part, this means that carbonados are more porous than single-crystal diamonds.

But this weird — one might even say Alien — crystal structure also makes carbonados extra useful for industrial purposes. Diamonds are really hard, and we like to take advantage of that fact by using them for cutting and drilling. But even diamonds can eventually crack or break, and then you have to replace the cutting surface with new diamonds.

However, carbonados have an advantage. Since they're aggregates, rather than uniform crystals, any break will be confined to that single piece, and not spread throughout. This makes carbonados as hard as diamonds, but much tougher.

Hardness is a material's ability to withstand abrasion, and here, carbonados match your typical diamond. Toughness, on the other hand, is how well a material resists shattering when force is applied, and that's where carbonados come out on top. Thus, they allow cutting and drilling tools to last longer and require less maintenance over time.

But the main reason scientists are interested in carbonados isn't what they look like or even what they can do — it's their origin. And the origin is… we don't know. The majority of ordinary diamonds form in the intense heat and pressure of the Earth's mantle, before eventually being blasted to the surface by volcanoes, and encased in rocks with characteristic mantle minerals.

But that doesn't seem to be the case with carbonados. They're not found in mantle rocks, and tend to include metals in their structure that wouldn't make sense if they formed in the mantle. Theories for how carbonados form include the heat and pressure of meteorite impacts, or possibly even radiation transforming carbon in the mantle.

However, neither method produces diamonds large or numerous enough to account for existing carbonados. Another contender is subduction zones, boundaries where tectonic plates collide and get shoved down into the mantle. The pressure and heat of this process would be plenty sufficient, but again, we should find mantle minerals present alongside carbonados.

All this leads scientists to suspect that carbonados must come from interstellar space. The crystals could form out there in some distant, carbon-rich spot, in the absence of Earth mantle minerals, and then get deposited here via asteroid. Plus, the space option could easily account for those head-scratching metal inclusions.

Furthermore, carbonados are only found in Brazil and the Central African Republic, which could be explained by an intense barrage of space rocks back when Africa and South America were part of the same supercontinent. All of this makes carbonados one of the few geological phenomena where the most likely explanation invokes outer space. Because once you've eliminated the impossible… well, you know.

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