A 555.55-carat black diamond called The Enigma is up for auction, and Sotheby's is accepting cryptocurrency in payment. Giuseppe CACACE / AFP via Getty images A 555.55-carat black diamond called The Enigma is up for auction. Sotheby's told Coindesk it will accept bitcoin, ethereum, and the stablecoin USDC in the auction. The Enigma was listed in the 2006 Guinness Book of World Records as the largest cut diamond in the world. Sotheby's is auctioning off a 555.55-carat black diamond, and crypto enthusiasts can bid for it using cryptocurrency. Dubbed The Enigma, the gem was listed in the Guinness Book of