Yolk greening is the greyish-green ring that forms around the yolk of a hard-boiled egg — a harmless but unattractive discoloration. It happens when eggs are overcooked or cooled too slowly: sulphur compounds from the egg white react with iron from the yolk at the interface, forming ferrous sulphide, the greenish compound. The chemistry is favoured by high heat, long cooking, and iron-rich yolks. Prevention is simple — boil gently rather than fiercely, limit cooking to about nine to twelve minutes depending on size, and plunge the eggs into cold water immediately to stop the reaction. The green ring is purely cosmetic: the egg is perfectly safe to eat, though the sulphurous smell of overcooked eggs is a further reason for restraint.