what causes green ring around yolk
1 GREEN --The greenish ring around a hard-boiled egg yolk is due to an iron and sulfur compound that forms when eggs are overcooked, not cooled quickly, or are prepared with water that is high in iron.2  Concentric green rings inside a cooked yolk are probably formed because the hen's feed or water contained iron. GREEN --The greenish ring around a hard-boiled egg yolk is due to an iron and sulfur compound that forms when eggs are overcooked, not cooled quickly, or are prepared with water that is high in iron.