There are lots of ways to cook a hard-boiled egg, and it can be surprisingly difficult. A seemingly simple task, achieving the perfect hard-boiled egg often comes down to timing. Get the timing wrong, and you can end up with anything from runny, undercooked eggs to drastically overcooked eggs with a rubbery white and chalky yolk. Then, there’s that green ring around the yolk that you might have seen when cutting into your hard-boiled eggs — in case you’re wondering whether those eggs are safe to eat, the answer is yes, hard boiled eggs with a green ring are perfectly safe to eat.
That green ring around the egg yolk is caused by a chemical reaction between the naturally occurring iron in the egg’s yolk and the sulfur in the egg’s white. When eggs are cooked for too long or at too high a temperature, the iron and sulfur interact to form ferrous sulfide, a greenish-gray compound, where the yolk and white meet. Although the green ring is harmless, eggs with a green ring won’t usually have the best texture, as they have been overcooked. They’re also less than ideal for dishes like a Tuna Niçoise salad or Cobb salad, where hard boiled eggs are quartered or sliced up and where the green ring will be noticeable.