A billion-year-old black diamond, believed to be the largest cut diamond in the world, has sold for $ 4.3 million.

The diamond, called the Enigma, is 555.55 carats and weighs almost as much as a banana.

Sotheby's auctioneer said "the buyer has chosen to use the cryptocurrency for the purchase".

There are several theories about the origin of the rock, including the theory that it was transported to Earth by an asteroid.

Sotheby's did not identify the buyer, but after the auction, cryptocurrency entrepreneur Richard Heart appeared on social media to claim he was the buyer of The Enigma. He told more than 180,000 of his followers on Twitter that "once payment was made and ownership was taken, the stone would be renamed the HEX.com diamond," referring to the blockchain platform he founded.

The stone is a carbonate, which is one of the most difficult forms of natural diamond. Carbonates are extremely rare and have only been discovered in Brazil and the Central African Republic. Because they contain osborin, a mineral found only in meteors, they are believed to have originated in space. However, the exact origin of black diamonds is shrouded in mystery.

Sotheby's described the Enigma as "one of the rarest billion-year-old cosmic wonders known to mankind."

Black diamonds are usually about 2.6 to 3.2 billion years old - that is, before dinosaurs existed.

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This billion-year-old black diamond, created by the explosion in space, goes on sale