At the boundary of the Earth’s mantle and core, materials are exposed to the extraordinary pressures of having both crust and mantle pushing down upon them. Temperatures of 4,000°C (7,232°F) have consequences as well. An effort to explore how common materials respond at those depths has revealed that the boundary produces an abundance of diamonds, while also oxidizing iron to rust.

“We both know what memories can bring,” Joan Baez sings, “They bring diamonds and rust.” The capacity of memories to convert charcoal to precious stones and shiny metal to worthless oxides might be familiar, but what goes on 3,000 kilometers (1,864 miles) beneath the Earth’s surface is less so. According to a paper in Geophysical Research Letters, however, the forces there make the metaphor real.

Using the Advanced Photon Source at Argonne National Laboratory, Dr Byeongkwan Ko, then a PhD student at Arizona State University, replicated these forces (albeit on a tiny scale) and applied them to iron-carbon alloy (Fe₃C) and water.

Despite the extreme differences from atmospheric pressure, they found the interaction produces familiar iron oxides and iron hydroxides – better known as rust. Meanwhile, these pressures also push the carbon out of the alloy and turn it into diamonds.

“Temperature at the boundary between the silicate mantle and the metallic core at 3,000 km depth reaches to roughly 7,000 °F, which is sufficiently high for most minerals to lose H₂O captured in their atomic scale structures,” said senior author Professor Dan Shim in a statement. “In fact, the temperature is high enough that some minerals should melt at such conditions.”

Carbon bonds strongly with iron, so it is thought there would be quite a lot of it in the iron-rich core. The mantle has also been found to be surprisingly rich in carbon, which scientists haven’t previously been able to explain.

The authors believe that wherever hydrogen is abundant near the core-mantle boundary, it alloys with the outer core’s liquid iron, forcing out other light elements including carbon. Their work indicates that when carbon escapes the core, it is turned into diamonds upon reaching the mantle. 

“Diamond formation at the core-mantle boundary might have been going on for billions of years since the initiation of subduction on the planet.” Ko said. Quite a lot of the Earth’s carbon may be sitting in diamond form near the boundary.

Diamonds are one of those minerals that can be formed by multiple processes under different conditions. These ones are safely beyond the reach of any miners. However, the authors think their presence could explain the carbon-in-the-mantle mystery. Moreover, they are hopeful their work can be verified by studying the speed of seismic waves.

“The reason that seismic waves should propagate exceptionally fast through diamond-rich structures at the core-mantle boundary is because diamond is extremely incompressible and less dense than other materials at the core-mantle boundary,” Shim said. 

The authors hope they can repeat this work by studying how other abundant elements respond to such conditions, expanding our knowledge of the composition of the deep mantle. 

Baez’s lyrics might be vindicated by science, but her song-writing contemporaries don’t come out so well. It seems the diamonds are not in the sky, with or without Lucy, but far beneath our feet, where contrary to claims, rust does indeed sleep.