A green or gray ring around the yolk of a hard-boiled egg is a very common and harmless result of overcooking. It does not mean the egg has gone bad or that it is unsafe to eat. Instead, the discoloration appears because of a natural chemical reaction that takes place when eggs are exposed to heat for too long.
Inside the egg, sulfur compounds in the white react with iron in the yolk when the temperature becomes too high or the cooking time is too long. This reaction creates iron
