'Extremely rare' 555.55-carat black diamond going up for auction

An "extremely rare" black diamond — believed to have come from outer space — is going up for auction next month in London. 

Sotheby’s Dubai on Monday unveiled the 555.55-carat black diamond, called "the Enigma." It is going on tour in Dubai and Los Angeles before it is due to be auctioned off in February in London. 

The auction house expects the diamond to be sold for at least 5 million British pounds ($6.8 million). It plans to accept cryptocurrency as a possible payment as well.

"To have a natural faceted black diamond of this size is an extremely rare occurrence and its origins are shrouded in mystery – thought to have been created either from a meteoric impact or having actually emerged from a diamond-bearing asteroid that collided with Earth," Sotheby’s said in a statement.

The design of the diamond is significant as well, inspired by the Middle Eastern palm-shaped symbol of the Khamsa — "a sign of protection, power and strength," Sotheby’s said.

The number five also bears an important significance to the diamond. It is not only 555.55 carats in size but also contains exactly 55 facets, a technical feat for one of the toughest diamonds in existence.

Black diamonds, also known as carbonado, are extremely rare. They are found naturally only in Brazil and Central Africa — two regions once connected as part of the supercontinent and believed to have been struck by meteorites, the auction house said.

They contain traces of nitrogen and hydrogen that are abundant in interstellar space, as well as osbornite, a mineral uniquely present in meteors.

Black diamonds are "one of the hardest materials known to man" and are difficult to cut and polish. 

"The Enigma – a natural faceted stone of record-breaking size and quality is therefore truly unique, marking an important moment for black diamonds at auction," Sotheby’s said.

Other items up for auction have made headlines recently, including a rare copy of the U.S. Constitution and a single page of artwork from a 1984 Spider-Man comic book that sold for a record $3.36 million.

This story was reported from Cincinnati.