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.