Origin of gems revealed

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.

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.

Based on findings at the Carnegie Institution for Science in Washington, it’s clear that blue diamonds are formed at far greater depths than other diamonds, some deeper than 410 miles, Smith said. Most other diamonds are formed between about 90 to 125 miles, said researcher Evan Shirey.

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.