The world’s biggest uncut diamond, ‘The Enigma,’ sold for $4.3 million.

London’s Sotheby auction house sold the diamond online on Wednesday. The 555-carat diamond sold quickly to an unnamed bidder for $4 million in cryptocurrencies.

This carbonado diamond is unusual. Unlike diamonds, carbonados contain “a unique combination of physical and chemical features,” according to National Geographic. Carbonados are exclusively found in Brazil and Central America.

One of the largest carbonados ever unearthed was the Sergio, a 3,167-carat diamond in Brazil.

Even more uncommon, carbonados’ origins are unknown, leading to several speculations regarding how they are generated.

NDTV reports that the Enigma’s main premise is that a meteorite or asteroid impacted Earth about 2.6 billion years ago. A meteor might have caused chemical vapour deposition, forming the diamond, or a diamond-bearing asteroid could have struck with Earth and landed near the surface.

The final owner of the Enigma and their motives for auctioning it have been kept secret for 20 years.

Shortly before the sale, Dubai, Los Angeles, and London displayed the black diamond. This came three years after specialists converted the unpolished stone into a 55-face treasure.

Carbonados are one of the toughest diamonds, and cutting into the Enigma proved difficult for specialists.

The diamond’s new form was inspired by the Hamsa, a Middle Eastern palm-shaped symbol of protection and the number five (suitable considering its 555 carats and 55 facets), according to NDTV.

“Most diamonds are cut into one of 10 popular shapes, but the Enigma resembles a hand,” Tobias Kormind, managing director of Mayfair, Europe’s biggest online jeweller, told NDTV.

In January, we revealed that the Enigma will be auctioned for $6.8 million. Though it sold less, the diamond is still one of the most costly diamonds ever auctioned.