You’ve probably been there: you slice into what you thought was a perfect hard-boiled egg… only to find a grayish-green ring hugging the yolk. It’s not exactly the appetizing look you were going for, and you might wonder if it’s a sign the egg has gone bad.

Good news — it hasn’t. That ring is the result of a completely harmless chemical reaction, and once you understand it, you’ll know exactly how to prevent it next time.

The Science Behind the Ring

The culprit is a compound called ferrous sulfide. The yolk of an egg contains iron, while the white (albumen) contains hydrogen sulfide. When eggs are cooked for too long or at too high a temperature, these two elements meet at the yolk’s surface and react, forming the telltale greenish ring.

It’s pure kitchen chemistry — not a freshness issue.

How to Prevent It

The trick is to avoid overcooking and cool the eggs quickly. Here’s a foolproof method for perfect hard-boiled eggs every time:

• Place eggs in a single layer in a saucepan and cover with at least 1 inch of cold water.
  Bring the water to a full rolling boil.
• Turn off the heat immediately, cover the pot, and let the eggs sit for 12–15 minutes.
• Transfer eggs to a bowl of ice water or run them under cold water for several minutes.

This gentle cooking method prevents excess heat from triggering the reaction — and the quick cooling stops the cooking process while making the eggs easier to peel.