WASHINGTON — Blue diamonds — such as 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.
“The origin of blue diamonds is such an alluring question — you don’t see them very often. And the famous ones, like the Hope diamond, have such mystique,” said Kim Tait, a gem expert at the Royal Ontario Museum in Toronto.
Just 1 out of 200,000 diamonds are blue. 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 (660 kilometers), Smith said. Most other diamonds are formed between about 90 to 125 miles (150 to 200 kilometers), Shirey said.
The researchers suggest that boron in the ocean floor was pushed down when plates that make up the Earth’s crust collided. The element allows the stone to absorb some red light, so the diamond looks blue.
Their findings were published Wednesday in the journal Nature.
“Most of the impurities contained in these blue diamonds came from a very deep source,” said Dongzhou Zhang, a scientist at the University of Hawaii, who was not involved in the research. “This study tells us for the first time that blue diamonds are formed very deep in the interior of Earth.”
Diamond deposits eventually reach the surface through volcanic eruptions.
The 45-carat, walnut-size Hope diamond, long rumored to carry a curse, wasn’t included in the study but the findings help us know where it came from, said the Toronto museum’s Tait, who had no role in the research. The Hope diamond’s many owners included Marie Antoinette.
“It’s such an amazing stone,” Tait said.