Why does a green ring appear around hard-boiled eggs?

You’ve made hard-boiled eggs for your salad, and then… surprise: a green circle surrounds the yolk. It’s common, but far from tasty. Despite the fact that you followed your instructions exactly, why does this minor inconvenience keep occurring? And, above importantly, how do you avoid it without becoming a Michelin-starred chef? Here are all of our tips for perfect hard-boiled eggs that taste as delicious as they look.

The culprit? An unexpected chemical reaction

That iconic greenish ring isn’t the result of a poor egg or a failed cooking attempt. It’s merely a reaction between the iron in the yolk and the sulfur in the white that results from overcooking. The end outcome is a little layer of iron sulfide, which is nontoxic but visually unattractive. Fortunately, a few changes are sufficient to eliminate this issue.

Quick Tip: Hard Boiled Eggs (without that gray-green ring around the yolk) – Diane's Food Blog