When egg whites cook, a small amount of hydrogen sulfide (H₂S) is released from sulfur-containing amino acids like cystine or methionine. If the H₂S migrates to the yolk, it combines with iron atoms to produce ferrous sulfide (FeS), which is a dark-colored material that looks green mixed with the bright yellow yolk. Ferric sulfide (Fe₂S₃) is also sometimes claimed to be formed in this process, likely because it is itself a yellow-green substance. There is, however, less data to back this up, aside from the convenient color match.