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The world's largest and most valuable blue diamond is going up for sale.

It's called The De Beers Cullinan Blue and experts say it could sell for more than £35million when it goes up for auction in April this year.

The diamond comes from South Africa which is one the few places in the world where you can find extremely rare blue diamonds.

Diamonds are weighed in units called carats, and whilst most blue diamonds are less than 10 carats - this diamond weighs a huge 15.1 carats!

The sale will take place at Sotheby's auction house in Hong Kong and experts say the jewel is remarkable on many levels.

Frank Everett from Sotheby's said: "It's rare because of the size. It's over 15 carats. It's a vivid blue. It's internally flawless. And really one of the most rare aspects of it is the cut."

"I think we're going to see tremendous interest in this stone when it finally comes to auction," he added.

It is thought that blue diamonds, unlike most other diamonds, are formed in the lower part of Earth's mantle and the presence of small amounts of a chemical element called boron gives them their special colour.

Only five blue diamonds weighing over 10 carats have ever gone to auction and it will be the first time that a diamond over 15 carats has gone up for sale.

The current record for the most expensive blue diamond ever sold is held by a stone known as the Blue Moon of Josephine which was bought for £36.7million in 2015.