Some diamonds—including large, exceptional gems similar to the Cullinan, Constellation, and Koh-i-Noor—carry with them remnants of the metallic liquid from which they formed deep in Earth’s mantle, according to new research (Science 2016, DOI: 10.1126/science.aal1303). The findings confirm predictions from theoretical and laboratory studies that Earth’s deep mantle could have highly reducing regions capable of producing iron alloys, but for which researchers lacked direct evidence. By using Raman spectroscopy, X-ray diffraction, and scanning electron microscopy, a team led by Evan M. Smith of the Gemological Institute of America studied a particular class of diamonds that are large, inclusion-poor, relatively pure, and irregularly shaped, and that show evidence of partial dissolution as they rose to Earth’s surface. The team found that diamonds of this particular class contain inclusions of a metallic mix of iron, nickel, carbon, and sulfur surrounded by a thin fluid jacket of methane and hydrogen. That combination of materials indicates that the diamonds formed from a metallic liquid in a reducing environment, the team explains.