The largest cut diamond in the world is about to go up for auction. The extraordinary jewel, nicknamed "The Enigma" is a billions-year-old Fancy Black diamond that is rumored to originate from space. According to the auction house, the stone is a carbonado black diamond, which are extremely rare, occur naturally, and usually date from 2.6 to 3.8 billion years old. It contains nitrogen and hydrogen but also a mineral called osbornite usually found in meteorites, which suggests an outer space connection. What that connection is exactly is unclear, however. Extraterrestrial diamonds from meteorites are tiny, usually nano-sized. This diamond...