You have probably seen it happen yourself. You are making a fresh batch of hard-boiled eggs to put in a salad, as a snack, or for deviled eggs, and when you cut one open, there it is: a dull green ring around the yolk like a bruise on a piece of meat.

Although such an effect might appear to be an indication of something having gone wrong, the reality is not so bad and even easier to correct than you might imagine.