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 sulfide, which forms the greenish or grayish layer around the yolk.
This usually happens when eggs are cooked at a strong rolling boil instead of a gentler simmer. The intense heat causes the proteins in the egg white to break down more than necessary, releasing sulfur gases. Those gases move toward the yolk, where they combine with the iron and create the discolored ring. Although it may not look very appealing, it does not change the egg’s safety and does not make it harmful to eat.
