Ever since it was found in South Africa in 1905 experts have been puzzling over how the Earth created a gem of such clarity. All other diamonds are smaller or chemically less pure.
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Now scientists have found that the Cullinan’s origins may have been as spectacular as its current setting. Their research suggests it was born four times closer to the centre of the Earth than other diamonds — and was then shot to the surface in an explosive volcanic eruption.
“Our research suggests diamonds like the Cullinan form in very extreme conditions, where bubbles of molten metal are trapped in red hot rocks up to 600 miles deep in the Earth,” said Evan Smith, at the Gemological Institute of America, whose research is published in Science journal.
Such conditions include temperatures of 1,500C and pressures of 250,000 atmospheres. “This is far deeper than where common diamonds form,” Smith said. “At this depth Earth makes diamonds to a different recipe — so they grow very pure and large.”
Diamonds such as the Cullinan are made of carbon that first dissolves into the molten metal and then coalesces into giant crystals. The Cullinan is also old — it reached the surface 1.2bn years ago and formed up to 2bn years before that, meaning it could predate life itself.
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Adrian Jones, of University College London, where he researches diamond formation, said giant diamonds such as the Cullinan may even be partly extraterrestrial in origin. “Diamond was around before our planet formed, and was part of the building blocks of Earth. Possibly some gem diamond grew using these exotic remnants as seeds. Perhaps some even contain a tiny remnant of non-terrestrial diamond at their centre.”
