DUBAI, United Arab Emirates— Auction house Sotheby’s Dubai has unveiled a diamond that’s from out of this world. Sotheby’s calls the 555.55-carat black diamond — believed to have come from outer space — “ The Enigma.” The rare gem was shown off Monday to journalists as part of a tour in Dubai, United Arab Emirates, and Los Angeles this month before it is due to be auctioned off in February in London. Sotheby’s expects the diamond to be sold for at least$ 6.8 million.... The auction house plans