People use the saying “out of this world,” to praise something as exceptional. But, at auction house Sotheby’s Dubai, officials are using the term literally. The company announced this week that it will sell a black diamond that they say is believed to have fallen to Earth from outer space. That would make it a literal “out of this world” jewel. Sotheby’s calls the large black diamond, “The Enigma.” It weighs 555.55 carats. A carat is a term of measurement for valuable stones often used in jewelry. One carat equals 200 milligrams. The rare stone was shown to reporters at Sotheby’s on Monday. In February, it will be put up for sale at Sotheby’s in London. Sotheby’s expects the purchase price will reach at least $6.8 million. Experts at the National Space Foundation in the United States call black diamonds “carbonado diamonds.” On its website, the science organization writes that carbonado diamonds form during supernova explosions. “Black diamonds,” the website says, “were once the size of asteroids a kilometer or more” across when they first began to hit Earth. Carbonado diamonds are extremely rare and are found naturally only in Brazil and Central Africa. Experts say study of the stone’s chemical structure leads them to believe they come from space.