A billion-year-old black diamond, believed to be the world's largest cut diamond, has sold for £3.16m ($4.3m).

Named The Enigma, the 555.55 carat gem, which weighs about the same as a banana, had been expected to fetch more than £4.4m in the online action.

Auctioneer Sotheby's said "the buyer has opted to use cryptocurrency for the purchase."

There are competing theories about the origins of the stone, including that it was carried to Earth by an asteroid

Sotheby's did not identify the purchaser but after the auction cryptocurrency entrepreneur Richard Heart took to social media to claim that he was the buyer of The Enigma.

He told his more than 180,000 Twitter followers that "as soon as the payment's gone through and possession's been taken" the gem would be renamed the "HEX.com diamond", in reference to the blockchain platform he founded.

The gem is a carbonado, which is one of the toughest forms of natural diamond.

Carbonados are extremely rare and have only ever been discovered in Brazil and the Central African Republic.

Because they contain osbornite, a mineral found only in meteors, they are believed to originate from space.

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

Although the precise origin of black diamonds is shrouded in mystery.

Black diamonds are usually around 2.6 to 3.2 billion years old - a time before dinosaurs existed.

The Earth itself is around 4.65 billion years old, so not much older than black diamonds.