How Did Blue Diamonds Get That Way? We Think We Know

Blue diamonds are the rarest kind. Roughly one in every 200,000 diamonds has the tint that makes the Hope Diamond so magical and made us swoon over Kate Winslet in Titanic.

A new study in the journal Nature gives us the best idea yet how it happens, and how they got here.

Researchers analyzed 46 blue diamonds (including one that sold for $25 million a couple of years ago) and concluded that the gemstones originated up to 400 miles below the Earth’s surface, as much as four times the depth of a clear diamond.

Diamonds get their blue color from boron, a chemical element mostly found on the Earth’s surface, when it combines with tiny mineral impurities in the diamond. But diamonds are formed deep underground, which has kept scientists wondering just how the boron got there.

Research coordinated by the Gemological Institute of America (GIA) led to the conclusion that tectonic plates collided eons ago and one slid below the other, carrying boron far deeper under the surface than normal.

“Blue diamonds like the famous Hope Diamond have intrigued scientists for decades,” said Evan Smith, a GIA research scientist and the study’s lead author. “The opportunity to study these rare diamonds at GIA gave us insight into their incredible origin.”

Jeffrey Post, curator of the mineral collection at the Smithsonian National Museum of Natural History, called the Nature study a “very compelling argument” that blue diamonds come from so deep in the Earth.

You can thank volcanic flows, by the way, for bringing them up to where we can get at them.

“We always knew there was something special about these diamonds,” Post said.