A 555.55-carat black diamond with interstellar origins could fetch upwards of $6.8 million at Sotheby's London next month.

The fascinating gem was revealed to journalists at Sotheby's Dubai on Monday. It was the first stop of a publicity tour that will continue to Los Angeles (Jan. 24-26) and then circle back to London for the online sale (Feb. 3-9).

Billed as the largest faceted diamond of any kind to appear at auction, "The Enigma" will be offered in its own dedicated online sale — without reserve. In auction parlance, that means the highest bid will be the winning bid, with no minimum required. Sotheby's will be accepting cryptocurrency, a strategy aimed at attracting younger, digitally savvy buyers.

The Enigma is considered a "carbonado diamond," which was likely formed in outer space and brought to earth by meteorites, according to scientists. Earth-formed diamonds often display a crystal structure, while carbonado diamonds — in their natural form — have an irregular surface that resembles charcoal.

Carbonado diamonds are found in only two places on earth — Brazil and the Central African Republic. (During the earth’s history, the eastern coast of Brazil may have been connected geographically to the western coast of Africa).

Black diamonds are also different than other colored diamonds because they do not get their color from trace amounts of nitrogen, hydrogen or boron, in the gem's chemical makeup. Instead, black diamonds owe their color to numerous dark inclusions (mostly graphite). Their opaqueness is caused by a “polycrystalline” structure that inhibits the reflection of light.

The Enigma officially entered the Guinness Book of World Records in 2006 as the "Largest Cut Fancy Black Diamond." Since then, it's been mostly out of the public eye.

The rare black diamond is awash in 5s, from its 555.55-carat weight to its 55 facets. The repetitive use of the number five is culturally significant in the Islamic world, where it is considered sacred and representative of the five Pillars of Islam.

Sophie Stevens, a jewelry specialist at Sotheby’s Dubai, told the Associated Press that the shape of the diamond is based on the Middle-Eastern palm-shaped Khamsa, a symbol that stands for strength and protection.

The auction house estimated that The Enigma would sell for £5 million ($6.8 million).

Credits: Images via Instagram / Sothebysjewels; Instagram / Sothebys.