Question: Why Do Egg Yolks Turn Green?
Have you ever had a hard boiled egg that had a green yolk or a yolk with a green to gray ring around it? Here's a look at the chemistry behind why this happens.
Answer: The green ring forms when you overheat the egg, causing hydrogen and sulfur in the egg white to react and form hydrogen sulfide gas. The hydrogen sulfide reacts with iron in the egg yolk to form a grayish-green compound where the white and yolk meet. While the color isn't particularly appetizing, it's fine to eat. You can keep the yolk from turning green by chilling the eggs as soon as they have finished cooking. One way to do this is by running cold water over the hot eggs as soon as the cooking time has elapsed.