Why do egg yolks turn green when cooked?
If you want a hard-boiled egg, it’s quite easy to end up with a green ring between the white and the yolk. This isn’t something that should worry you, since the egg is still perfectly safe to consume. This happens because you boiled the egg for way too long. And in this case, the hydrogen in the egg white combines with sulfur in the yolk, resulting in this green color.