They are the world’s most expensive diamonds, with some stones valued at £100 million.
But until now nobody has known how rare blue diamonds are made or where they come from.
Now scientists have discovered that they are formed 400 miles down in the Earth, around four times as deep as clear diamonds, where the element boron combines with carbon in such extreme pressure and heat that it crystallizes into the world’s most precious stone.
And because boron is mostly found on the Earth’s surface, scientists believe that it must have travelled down into the mantle when tectonic plates slipped beneath each other.
Eventually volcanic action brought the diamonds up closer to the surface.
The study, published in the journal Nature, suggests blue diamonds are even rarer than first thought.
“We now know that the finest gem-quality diamonds come from the farthest down in our planet.” said Steven Shirey, of the Carnegie Institution of Science.
Blue diamonds have always held a special intrigue. The world’s most famous jewel, the Hope Diamond, which was once owned by Louis XIV, Marie-Antoninette, and George IV was said to be cursed with many of its owners and their families coming to a sticky – and often headless – end.
The postman who delivered the Hope Diamond to its current location in the National Museum of Natural History in Washington DC had his leg crushed in a lorry accident shortly after and then his house burned down.
But the value and rarity of blue diamonds makes them difficult to study and researchers at the Carnegie Institution have spent two years tracking down and studying 46 blue diamonds from collections around the world.
And they were looking for the rarest of blue diamonds, those which include tiny mineral traces called inclusions which hint at their origins.
“These so-called type IIb diamonds are tremendously valuable, making them hard to get access to for scientific research purposes,” said lead author Evan Smith of the Gemological Institute of America, adding,
“And it is very rare to find one that contains inclusions, which are tiny mineral crystals trapped inside the diamond.”
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