Blue diamonds get hue from sea floor

THE HOPE DIAMOND, a rare blue diamond that is one of the world’s most famous jewels, has had a complicated history, passing through the hands of monarchs and bankers and heiresses and thieves before landing for all to see at a Washington museum.

The geological history of blue diamonds is even more complex, according to research published yesterday examining these exceptionally scarce and valuable gems.

Scientists analysed 46 blue diamonds, including one from South Africa that sold for $25 million (R328.6m) in 2016, and determined that they can form at depths of at least 660km, reaching into a part of the earth’s interior called the lower mantle. Tiny mineral fragments trapped inside them provided clues about the birthplace of the diamonds.

Blue diamonds comprise only about 0.02 percent of mined diamonds, but include some of the world’s most famous jewels.

Diamonds are a crystalline form of pure carbon, forming under enormous heat and pressure. Blue diamonds crystallise alongside water-bearing minerals that long ago were part of the sea floor, but were shoved to great depths during the inexorable movement of the immense tectonic plates, the researchers said.

Scientists already knew these diamonds acquired their blue hue from the element boron. This study indicated this boron once had been in ocean water and was incorporated into the sea floor rock.

“This is the first time anyone has come up with a factbased story or model for how blue diamonds form. 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,” said Gemological Institute of America research scientist Evan Smith, who led the study published in the journal Nature.