Massive diamonds show the Earth’s mantle contains globs of liquid metals

By Katherine Ellen Foley

Everyone’s favorite form of carbon sure is sparkly. And now, scientists have evidence that some of the largest natural diamonds come from deep down in the Earth, surrounded by blobs of hot, liquid metal.

Diamonds are made up purely of carbon crystals that form under high pressure and temperature. At least theoretically. In reality, most of the gemstones we see in jewelry have some sort of chemical imperfection preserved within them—like a nitrogen atom substituted for a carbon atom in the crystal formation, or perhaps a bit of graphite. Studying these imperfections can give geologists clues about the conditions of the environment beneath the Earth’s surface, where these gems form.

At present, scientists can only look at the gemstones that make it to ground level inside kimberlite, magmatic rocks that form in the mantle, and bubble up to the surface through eruptions. The diamonds found inside kimberlite typically were formed roughly 100 miles (160 km) below the Earth’s surface.

But the world’s largest diamonds seem to be much different. In a recent study (paywall) published in the journal Science, a team of geologists analyzed 53 large diamonds and came to the conclusion that they formed within gobs of molten metals, much deeper than 100 miles below the surface. The study also provides the first direct evidence that these metallic liquids exist; before, they had only been theories.

Finding samples of large diamonds to study is a challenge in itself—most end up in the hands of jewelers before scientists can get their hands on them. So, according to NPR, the team had to rely on bits cut away from larger diamonds as part of the jewelry-prepping process. These diamonds were all class-II, which means they were similar to the Cullinan diamond, the largest diamond ever found, weighing over a pound when it was discovered in South Africa in 1905. The Cullinan diamond was over 3,100 carats; average engagement rings are usually 0.5 to one carat. (Since then, the Cullinan diamond has been cut into a number of other—still very large—stones.)

Within the diamond bits, they found evidence of a mix of metals, like iron and nickel, and thin layers of methane and hydrogen—but not oxygen. That, the researchers say, means the diamonds were formed inside blobs of liquid metal without any oxygen. This is the first-ever proof that oxygen isn’t as uniformly strewn about through the Earth’s mantle as scientists had previously thought

Additionally, some of these diamonds contained trace amounts of garnet, which means they could only have been formed under intense pressure. That, the scientists concluded, means the diamonds had to have been formed deep, deep within the Earth, where there’s much more gravitational pressure exerted on objects. They estimate that the diamonds came from somewhere between 200 and 500 miles below the Earth’s surface. Even at the low end, that’s 100 miles deeper than previous estimates.

The researchers think the diamonds’ deep origins can help geologists and chemists understand where metals form in the Earth, and how they cycle through the planet. If they can understand how metal circulates beneath our feet, they can better understand how our planet came to be what it is—and how it’s able to support life.