If your hard-boiled egg has a green-gray ring around the yolk, don’t worry—it’s not spoiled. The ring forms when eggs are overcooked or cooled too slowly. Heat causes sulfur in the egg white to react with iron in the yolk, creating a harmless compound called iron sulfide. The egg is still safe to eat, though the texture and flavor may be slightly affected. To prevent this, avoid overcooking and cool eggs quickly in ice water after boiling.