Diamonds that formed hundreds of kilometers deep below Earth's surface contain traces of chemical reactions that took place on the bottom of the ocean.

Given that the bottom of the ocean is just 11 kilometers (6.8 miles) down at its deepest point, this may seem rather odd – but those diamonds are a really valuable clue for understanding the exchange of material between Earth's surface and its crushing depths, researchers say.

"Nearly all tectonic plates that make up the seafloor eventually bend and slide down into the mantle – a process called subduction, which has the potential to recycle surface materials, such as water, into the Earth," explained geologist Peng Ni of the Carnegie Institution for Science.

Diamonds are prized for their beauty when shaped into faceted gems, but the little clear lumps of carbon can tell us a lot about the conditions in which they formed. Not all diamonds are perfectly clear; some contain what we call inclusions, fragments of other minerals that got caught up in the diamond formation process (and, once, a whole other diamond).

Sometimes, we can tell these inclusions are from the deep environment where the diamond formed; calcium silicate perovskite, for instance, is unstable at depths above about 650 kilometers except when it's been trapped in a diamond, so it's unlikely to have formed at the surface.

The diamonds studied by Ni and his colleagues were offcuts of large commercial diamonds dug up from deep underground, likely discarded for their impurities.

"Some of the most famous diamonds in the world fall into this special category of relatively large and pure gem diamonds, such as the world-famous Cullinan," said geologist Evan Smith of the Gemological Institute of America. "They form between 360 and 750 kilometers down, at least as deep as the transition zone between the upper and lower mantle."

Where commercial diamond companies see impurities, scientists see inclusions, and the team took this opportunity to study the planet's interior. Instead of deep minerals, though, the researchers found heavy isotopes of iron, outside the known values for iron from those mantle depths, or the products of reactions we'd expect at those depths.

This, they say, is the first evidence confirming a geochemical pathway to capture and transport surface materials deep into the mantle.

The key to this transportation is a mineral called serpentinite, so named because of its reptilian texture. The process that forms serpentinite is called serpentinization, and it happens when water atoms are added to the crystalline structure of peridotite.

Peridotite is typically found in Earth's upper mantle. For serpentinization to take place, the rock needs to be exposed to water. Under the ocean, this exposure happens when water travels down, or rock is pushed up, and the place in which both these things are most likely to happen are tectonic faults – where the edges of tectonic plates meet.

Well, "meet" is a mild way to put it. Usually, they overlap, with the edge of one tectonic plate being pushed beneath the other. Under the ocean, these are known as subduction zones, and there's a lot of serpentinite hanging around in them.

So where does the iron come into it? Serpentinite isn't the only material that emerges from the serpentinization process. One of the secondary materials – a sort of by-product – is iron-rich magnetite, and other iron-nickel alloys with a heavy iron isotopic composition.

This isotopic composition is very similar to the iron isotopic composition of the inclusions the researchers found in their two chunks of diamond.

This suggests, the team said, a fascinating cycle. Peridotite is exposed to seawater in a subduction zone, leading to the formation of serpentinite and its secondary materials. Then, some of these newly formed minerals seep back down through the crack between the tectonic plates and are carried hundreds of kilometers down, where they can sometimes get bound up in forming diamonds for scientists to find, millions of years later.

"Our findings confirm a long-suspected pathway for deep-Earth recycling, allowing us to trace how minerals from the surface are drawn down into the mantle and create variability in its composition," said geochemist Anat Shahar of Carnegie Science.

The team's results have been published in Science Advances.