Blue diamonds shine with a particular allure within the cultural zeitgeist. From Titanic’s fictional “Heart of the Ocean” to the real-life, possibly cursed Hope Diamond, these hued gems have captured the public’s attention, and their rarity arouses a sense of glamour. But exactly how these diamonds became shades of azure and sky hasn’t been exactly known. Earth scientists know the color is because of boron, but that chemical element is predominantly found on the Earth’s surface. So how did boron end up in gems formed 400 miles beneath ground?

In a study released Wednesday in the journal Nature, scientists posit a new theory explaining why blue diamonds are different from other kinds of diamonds. The answer comes down to the moving and shaking of oceanic plate tectonics. Blue diamonds, they posit, grew in the presence of rocks that were essentially once a part of the ocean floor. They hypothesize the boron found in blue diamonds came from the seafloor before it was shot down into the mantle — where diamonds are formed — when one tectonic plate slid beneath another.

“This is the first time anyone has come up with a fact-based story or model for how blue diamonds form,” explains study co-author and Gemological Institute of America research scientist Evan Smith, Ph.D., to Inverse. “Prior to this study, we had no idea where they form, what kinds of host-rocks they form in, or where they might be getting their boron from.”

blue diamond
A blue, boron-bearing diamond with dark inclusions of a mineral called ferropericlase.

The key to making this boron observation was by identifying the mineral inclusions inside blue diamonds — something no one else has done before. Inclusions are the small mineral fragments trapped inside a diamond as it grows that, according to Smith, “give a sort of snapshot of the birthplace of the diamond.” Overall, the team analyzed the trapped mineral grains in 46 blue diamonds. Initial analysis revealed the minerals, like ferropericlase could have only crystallized if they underwent the extreme pressure and temperature conditions of the lower mantle.

That indicated to the scientists that blue diamonds had to be “super-deep” diamonds that formed at a depth below the continental plates. Smith says that about 99 percent of diamonds originate within the continental plates. Comparatively, blue diamonds are forming at the extreme depths of the lower mantle, about 400 miles beneath the surface.

In turn, establishing that blue diamonds are “super-deep” allowed scientists to say that blue diamonds had to grow within oceanic plates forced down by ancient plate tectonic movement. Based on those two observations, they developed the hypothesis that the boron atoms that make blue diamonds blue originated at the surface and then were driven deep into the mantle by these sinking plates.

Hope Diamond
The Hope Diamond.

“This last part is important because it gives us a glimpse of a geochemical pathway capable of taking elements from the surface, such as boron, and recycling or transporting them deep into the Earth,” says Smith. “The boron we see is in all likelihood surface-derived, and the most likely explanation is that it hitched a ride with hydrous minerals in the down-going oceanic plate.”

If that’s true, Smith explains, it also suggests water can be carried down this deep, because the most likely vehicle for boron in the plate is water-bearing minerals, starting with serpentine. That’s cool news for Earth scientists because while it’s established that oceanic tectonic plates are subducted continuously, it hasn’t exactly been known what they drag down with them.