Perhaps you've hard-cooked an egg only to discover a green ring around the yolk when you crack it open. Here's what happened and what you can do to avoid it:
A greenish-gray ring may appear around a hard-cooked egg yolk. It's unattractive, but not harmful. The ring is caused by a chemical reaction involving sulfur (from the egg white) and iron (from the egg yolk), which naturally react to form ferrous sulfide at the surface of the yolk. The reaction is usually caused by overcooking, but can also be caused by a high amount of iron in the cooking water.
Eliminate the ring by avoiding overcooking and by cooling the eggs quickly after cooking. Run cold water over the just-cooked eggs or place them in a bowl or container of ice water until they have completely cooled. Then refrigerate the eggs in their shells until you're ready to use them. Hard-cooked eggs in the shell can be refrigerated up to one week. Check out tips for preparing hard boiled eggs from the American Egg Board.
Sources:
How to Make Hard-Boiled Eggs, American Egg Board
Doneness Guidelines, American Egg Board
