First, the green ring around the yolk is caused by a reaction between sulfur in the egg white and iron in the egg yolk. This usually happens when eggs have been cooked for too long or at too high a temperature, or if they're left to cool slowly. The green ring is not harmful to eat, but it can look a little icky, unless you are a fan of Dr. Seuss. Here's how to prevent it: Don't overcook the eggs: Place eggs in a single layer in a saucepan, and cover with about 1 to 2 inches of water. Slowly bring the water to a boil over medium heat. Once the water is boiling, let the eggs cook for 9-12 minutes. Quick cooling: As soon as the cooking time is done, immediately drain off the hot water, and plunge the eggs into a bowl of ice water or run them under cold water. The rapid cooling helps prevent the green ring from forming. Peel and use the eggs promptly: The green ring can sometimes form on eggs that have been cooked and then stored in the shell for several days. To avoid this, peel and use your hard-boiled eggs within a couple of days.