The green ring is a result of a simple chemical reaction between two compounds naturally found in the egg:
Iron (found in the yolk)
Hydrogen Sulfide (a sulfur gas released from the white when heated)
When an egg is cooked for too long or at too high a temperature, the heat drives the sulfur from the white to migrate toward the yolk. At the yolk’s surface, the iron and sulfur react to form ferrous sulfide—the harmless but unappealing greenish-gray compound.
