Have you ever wondered why when you open a hard-boiled egg, sometimes the outside of the yolk is green or greyish? The green ring around around a hard-boiled egg is caused by over cooking. Sulphur and iron compounds in the egg react during cooking to form ferrous sulphide on the surface of the yolk, causing it to turn greenish-grey. Although visually unappealing, ferrous sulphide is harmless and safe to eat. Here are 3 tips to avoid green yolks in the future. 1) Use fresh eggs. How do you know if your eggs are fresh? Click  here. 2) Don't over cook your egg. Cooking eggs (for hard boiled) more than 15 minutes will for sure result in a green yolk. Try our no-fail way to cook the perfect hard-boiled egg here. 3) Quickly cool down the eggs. Do this by running the eggs under really cold water for a few minutes. Sometimes I even add a few ice cubes to the water if I'm in a hurry for the egg to cool. Quickly cooling eggs also make them easier to peel.   Photography by Leah Kuhne