This huge black diamond just sold for $4.3 million. No one knows where it came from.

With a final closing bid of $4.28 million, an anonymous buyer just became the new owner of the 555.55-carat black diamond known as the Enigma. The sale of this bizarre stone has reignited a long-simmering debate about where it came from, stoked by the controversial theory that it may have arrived from outer space.

The Enigma and all other carbonado diamonds formed in a mysterious event some 3.8 to 2.6 billion years ago. Jet black, opaque, and full of visible holes, carbonados have a unique combination of physical and chemical features unlike any other known diamonds.

They are found in only two regions of the world—Brazil and the Central African Republic—and can grow astonishingly large. The group includes