Why do we observe a greenish ring around the yolk of hard-boiled eggs

Have you ever cracked open a hard-boiled egg thinking you’d overcooked it because a green ring surrounded the yolk? Don’t worry: this very common phenomenon is nothing to be alarmed about. And yet, it’s always a bit disappointing when you were hoping for a perfectly golden, crisp, and appetizing yolk. Fortunately, a few simple steps are all it takes to say goodbye to that greenish border and get eggs worthy of a Parisian brunch.
Why does this green circle actually appear?
The explanation is purely natural. When an egg is cooked for too long or at too high a temperature, two elements inside—the iron in the yolk and the sulfur in the white—react together. The result: a slight greenish deposit forms around the yolk.