Inside a secure laboratory in New York City geologist Evan Smith is peering into billion-year-old blue diamonds to gauge the inner workings of Planet Earth.

The cold, dense objects zap the warmth from his hands. They come in a range of hues. “Some are so pale you wouldn’t know they’re blue, some get a gray tone along with the blue, so they can look a little mysterious,” says Smith, who works in the diamond-grading lab at the Gemological Institute of America. Ultimately, it is the color—which is dependent on the amount of the element boron the gem captures—that holds the key to understanding Earth’s unknown depths. Smith has spent the last two years examining boron as well as other elements and minerals that the diamond ensnared as it formed, like insects fossilizing in amber.

With the help of a team of geologists and gemologists from Australia and the U.S., Smith traced the materials in nearly 50 blue diamonds to depths of roughly 400 miles beneath the ocean floor. For reference, the deepest humans have been able to drill into Earth’s surface is about seven miles. From the minerals found in these relics of Earth’s mantle, the team concluded the boron that gives these rarely studied diamonds its gripping blue color is the same as that found in the ocean floor. “It gives us a clue as to how the layers of the Earth are recycling,” says Megan Duncan, an Earth scientist at the University of California, Davis, who was not involved in the research. The team’s findings were published Wednesday in Nature.

Over the course of hundreds of millions to billions of years, the seafloor absorbs boron. As the floor becomes older and colder, it eventually becomes denser than the mantle beneath it and sinks. “It’s like a continuous conveyor belt that just keeps going for hundreds of millions of years,” says Lars Stixrude, head of Earth sciences at University College London who was not involved in the research. The boron, encompassed by rock that protects the mineral from the mantle’s high pressure and heat, continues on its path hundreds of miles downward until it reaches the lower mantle. There, the environment has such intense levels of heat and pressure that it melts boron’s protective rock sheath. Here in this deep-Earth pressure cooker, blue diamonds form. The process can take hundreds of millions of years—and that does not include the hundreds of millions of years the diamonds take to travel to the surface via volcano-like burrows called kimberlites (known to most as mines).

This process not only yields the gemstones that adorn ring fingers but also provides clues into what’s going on in the strata below Earth’s crust—an area that comprises 99 percent of the planet’s volume. “What happens at these levels is what drives the formation of plate tectonics and volcanoes, and the composition of our atmosphere,” Stixrude says. “It’s the unseen engine. Everything that we see on the surface is a result of what’s happening below.”

Boron is just one of the elements embedded in these diamonds that can help answer such weighty questions. The fact scientists can trace the origin of carbon-based gems like diamonds to such depths provides insight into how life as we know it came to be. “That piece of carbon is locked forever in this record of diamonds,” says Graham Pearson, a mantle geochemist at the University of Alberta who consulted with Smith during his research but was not formally involved in the study. Previous work by Smith and others published in Science and Nature has found evidence for the same ocean-to-mantle recycling system, but this most recent study is the first to examine these processes using blue diamonds.

Yet the depth at which these rare gems form could also hinder further understanding. Blue diamonds make up less than 0.02 percent of mined diamonds—and, because of their value, are extremely hard to come by for research purposes. The only reason Smith was able to study about 50 diamonds within two years was because of his post at the Gemological Institute. One of the diamonds Smith studied for the research measured at 24 carats and later sold for $25 million during an auction at Christie’s. For Smith, however, it is not the market value but the minerals trapped inside that make his heart race and mind drift when he examines a gem. “It’s almost like when you look inside the diamond, you’re going on vacation to a special place,” Smith says. He imagines an inner mantle where the heat emanating from the rocks would lend the area a red glow.

Since Smith and his team submitted their research to Nature in January, he has continued to examine blue diamonds that come through the institute. He hopes that in the coming months more advanced technology will enable him and his team to essentially “fingerprint” the boron’s composition to further confirm it is the same as that found in the ocean floor. This technology will have to be nondestructive, and will have to capture the unique type of the boron in the diamond.

The renewed energy and opportunity to investigate blue diamonds continues a pursuit of curiosity that has always surrounded the rare gems. “I cannot remember when I did not hunger after thrills,” wrote the late Evalyn Walsh McLean, a socialite famous for being the last private owner of the 45-carat, dark blue Hope Diamond now enshrined at the Smithsonian National Museum of Natural History in Washington, D.C. “We live just once, and of all the things in this world, I hate boredom most.” If only she had lived long enough to learn about her blue diamond’s journey from the mysterious depths of our planet into her hands.