The Cullinan mine, located on a diamond-bearing kimberlite pipe in the Gauteng province of South Africa, is the world’s richest source of rare blue diamonds and has produced more than a quarter of the world’s diamonds that are greater than 400 carats. The massive mine is also a scientific treasure trove. As scientists report in Nature on Wednesday, the Cullinan mine’s massive diamonds aren’t just a luxury item but a clue to what’s happening at the deep core of the Earth.

In the paper, a team of international researchers report the rare scientific discovery they found in the mine: a “super-deep” diamond encasing the mineral calcium silicate perovskite (CaSiO3), which is the fourth-most abundant mineral in the Earth but has never been found in nature until now. Super-deep diamonds, the researchers tell Inverse, are a classification reserved for those rare diamonds formed between 200 to 1,000 kilometers into the lower mantle, so they are super rare, too.

“This was very special because this mineral had been theoretically predicted, but it was not thought possible to see it preserved at the Earth’s surface for observation and measurement,” University of Alberta researcher and study co-author Graham Pearson, Ph.D., tells Inverse. Pearson is well-known as one of the world’s leading diamond researchers. “Finding a natural object that has never been seen by anyone before is always exhilarating! It’s what most natural scientists dream about.”

A cartoon illustrating the findings. Graham Pearson

Pearson and his team determined that the super-deep diamond originated around 760 kilometers below Earth’s surface — much deeper than most diamonds, which on average form between 150 to 200 kilometers below ground. Natural diamonds are formed when carbon molecules form highly organized “lattices” at high temperatures and pressure. Because this particular diamond was formed so far below the surface, Pearson estimates, it would have sustained more than 24 billion pascals of pressure. It was likely that it was able to incorporate the precious CaSiO3, which can only exist at the very high pressures close to the Earth’s crust. The inclusion of the precious CaSiO3 inside the diamond was confirmed by X-ray measurements made by the paper’s primary author, University of Padova professor of geoscience Fabrizio Nestola, Ph.D.

It’s very energetically difficult for minerals to rearrange their atoms to other structures, Pearson explains. So, once a diamond becomes a diamond, it’s not going to suddenly switch up its carbon lattice to turn into graphite, even though it might “want” to in the relatively low-pressure atmosphere of the Earth’s surface. This was a good thing for Pearson and his co-authors, because CaSiO3 is only stable at the extremely high pressure that exists over 500 kilometers below the Earth’s surface. There, it’s extremely abundant — the scientists estimate there could be 1,021 tonnes of perovskite in deep Earth — but in order for it to rise up to the surface, it needs a vehicle. Fortunately, it found one far closer to its home, the extremely hard, protective containers we know as diamonds.

“Only the super-strong nature of the diamond, and the particular nature of the fast eruption of the host kimberlite, in this case, provided a favorable set of circumstances that led to the preservation of this mineral,” says Pearson. “Many people predicted that we would never actually see a natural version of this mineral at the Earth’s surface because it is so unstable.”

The Cullinan mine in South Afica. Petra Diamonds

While the diamonds of the Cullinan mine are considered the world’s most commercially valuable, they’re also the most scientifically valuable. Diamonds, in general, are valued among geoscientists because they “provide access to the deepest intact material from the Earth’s interior through the minerals contained within their volumes,” the authors write.

In turn, the super-deep diamonds of Cullinan are so precious because they are some of the deepest physical samples of Earth’s interior ever found. Here, the perovskite structure within the diamond “very clearly” provides proof that as oceanic plates are pulled into Earth’s lower mantle, that crust transitions into a new mineral.

Next up for this diamond is further analysis by scientists at the University of British Columbia who will attempt to learn more about its age and origin. Understanding how the formation of super-deep diamonds differs from diamonds created at more shallow depths will help scientists paint a better picture of the dynamics and chemistry of the molten mess of minerals found deeply embedded in the planet’s mantle.