How the world's biggest sapphire grew so large

The world's largest ever blue sapphire has been unveiled, discovered in Sri Lanka's Ratnapura mines.

It weighs an astonishing 1404.49 carats, equivalent to about 280g (just under 10 ounces). Local gemologists say they have never seen a larger sapphire.

It is a type of gem known as a "star sapphire", due to the six-pointed star within the gem. This occurs because of the way light reflects from crystals trapped within it.

Its anonymous owner spoke to the BBC and suggests it could sell for up to $175m.

BBC Earth spoke to mineral scientist Simon Redfern from the University of Cambridge in the UK, to find out how such enormous gems can form.

Making sapphires is a lengthy process. This particular sapphire would have formed within the rocks of Sri Lanka's highlands.

The rocks are mostly granites, which form when molten magma cools and solidifies. But they have been subjected to intense heat and pressure.

"The granites have been dated as almost two billion years old, and were subsequently squeezed and re-worked in a massive metamorphic mountain-building episode that happened more than 500 million years ago," says Redfern. "Temperatures and pressures deep within the roots of these mountains would have reached more than 900C and over 9000 atmospheres pressure."

The sapphire could have formed within the granite itself before it was re-worked, or later when the rocks were being heated and squeezed.

"In either case, the temperatures and pressures would have changed only very slowly over millions and millions of years," says Redfern.

"This is how the crystal was able to grow so big."

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