Why Do my Hard-Boiled Eggs Have a Green Ring around the Yolk?

That greenish-gray ring around an egg yolk is not attractive, but what is it, and is it safe to eat?

One of the most common questions I get about cooking eggs is about the green ring around the yolk that sometimes happens when eggs are hard boiled.

It's certainly not pretty, and not something you want to see when you're making hard-boiled eggs, deviled eggs, or egg salad, but what exactly is it? 

What causes the green ring? 

And is it safe to eat?

Why Do my Hard-Boiled Eggs Have a Green Ring around t...