The rarest diamonds on Earth were forged hundreds of miles below its surface, scientists announced on Thursday.

A team of geologists and gemologists from Australia and the U.S. analyzed 46 blue diamonds, including one from South Africa that sold for $25 million in 2016, to determine that the precious gems are formed at depths of 410 miles below the Earth’s surface. The findings were published in the journal Nature.

For perspective, the International Space Station orbits about 250 miles above the Earth and the deepest humans ever have drilled below the surface is about seven miles.

Blue diamonds comprise about 0.02 percent of mined diamonds, according to experts, but they include some of the world’s most exquisite jewels. Their origins long have been shrouded in mystery.

“We knew essentially absolutely nothing about where they grow,” geologist Evan Smith, a lead author of the report and a research scientist at the Gemological Institute of America in New York, told the Washington Post.

The color of these rare gems tipped off scientists to how they were formed. Their striking blue hue, which is partly dependent on the amount of the element of boron the gem captures, is helping scientists to unlock the mysteries deep in the planet's core.

A blue, boron-bearing diamond that was examined as part of the study announced in Nature. (Evan Smith/Gemological Institute of America)

“We always knew there was something special about these diamonds,” geologist Jeffrey Post, curator of the mineral collection at the Smithsonian's National Museum of Natural History, who was not involved with the Nature report, said in the newspaper. Post added that the authors make a “very compelling argument” that these diamonds formed at greater than typical depths.

'QUADRILLION' TONS OF DIAMONDS FOUND DEEP BELOW EARTH'S SURFACE

The scientists concluded that the boron coloring the diamonds is the same as the boron found in the ocean's floor. However, that spurred more questions.

As the sea floor ages and becomes colder, it eventually becomes denser than the mantle beneath it and sinks. The boron, which is encased by protective rock, heads miles underground to the lower mantle.

That deep, hidden area of extreme heat and pressure is where rare blue diamonds are formed. And they still must make a journey of millions of years back toward the surface.

“It gives us a clue as to how the layers of the Earth are recycling,” Megan Duncan, an Earth scientist at the University of California, Davis, who was not involved in the research, told Scientific American.