A black space diamond, estimated to be more than a billion years old, has been sold to a cryptocurrency millionaire for $4.3 million. The diamond is the largest of its kind, weighing 555.55 carats.

British auction house Sotheby's confirmed the sale Thursday, stating that the extremely rare black diamond was bought by someone using cryptocurrency. The company did not outright name the buyer in its announcement.

 Entrepreneur, YouTuber, and founder of Hex.com, Richard Heart, later posted a tweet claiming that he had bought the diamond. Heart, who owns the world's first high-interest blockchain certificate of deposit and PulseChain network, stated that he will be renaming the diamond as the "Hex.com Diamond." The diamond was previously referred to by Sotheby's as "The Enigma."

The black diamond, which is believed to have come from space and deposited on the earth more than a billion years ago, was recognized by the Guinness World Book of Records as the world's largest cut diamond. In its brochures, Sotheby's referred to the diamond as one of the world's rarest discoveries known to humankind.

The diamond is a type of mineral called a carbonado diamond. It is typically found in deposits on the earth's surface, but rarely if never the same size as the Enigma. Scientists believe that these types of diamonds were likely created by massive meteoric impacts either from material expelled from supernova explosions or rare diamond-bearing asteroids.

 Unlike normal diamonds, carbonado diamonds contain trace elements such as nitrogen and hydrogen, which are abundant in space. It also contains a mineral called osbornite, which is commonly found in meteors. Carbonados are made up of microscopic diamond grains that are irregularly oriented, absorbing more light and giving them their dark look.

Carbonado diamonds are presently only found in Brazil and in Central Africa, which was once linked for more than a billion years as part of the Rodinia supercontinent. This, according to Sotheby's, leads to the hypothesis that carbonados came from space, as the area was thought to have been bombarded by meteorites while they were still attached a billion years ago.

Sotheby's said the unique structure of carbonado diamonds makes cutting and polishing them virtually impossible, and it would take specialized tools as they are harder than normal diamonds. The company added that it took them more than three years to shape the Enigma diamond into its final shape. "The Enigma" was said to have weighed over 800 carats when it was first discovered.