Insight on how blue diamonds were created

WASHINGTON — Blue diamonds — like the Smithsonian’s famous Hope diamond — are the rarest of all, and how they formed more than a billion years ago is a bit of a mystery. Now scientists think they have a glimmer of an answer.

They’ve long known that the blue tint comes from traces of boron in the diamond. But the element is mostly found near the Earth’s surface, not deep down where diamonds are typically created.

Researchers scrutinized 46 blue diamonds, studying imperfections in the gems for clues.

Like all diamonds, they are made when carbon comes under intense pressure and extreme heat deep inside the Earth. As they form, they can trap tiny bits of rock inside — like fossils in amber.

“Diamond is an extraordinary container, a time capsule,” said Steven Shirey, a geochemist at the Carnegie Institution for Science in Washington.

Shirey and his colleagues used lasers to examine the diamonds’ imperfections — slivers of embedded rock — at the Gemological Institute of America. How the light is reflected helps identify the minerals inside, said Evan Smith, a research scientist at the institute who led the study.

“You can start putting together a picture of what the surrounding rocks looked like, to recreate the birthplace of the diamonds,” Smith said.

Based on their findings, it’s clear that blue diamonds are formed at far greater depths than other diamonds, some deeper than 410 miles, Smith said.

The researchers suggest that boron in the ocean floor was pushed down when plates that make up the Earth’s crust collided.

Their findings were published Wednesday in the journal Nature.

The 45-carat, walnut-size Hope diamond, long rumored to carry a curse, wasn’t included in the study.