You peel a hard-boiled egg, slice it in half, and notice a faint greenish-gray ring around the bright yellow yolk. Before you worry, there’s no need to panic. That ring is harmless and doesn’t mean the egg has gone bad. It’s simply the result of a natural chemical reaction that happens during cooking.

The greenish ring forms when sulfur from the egg white reacts with iron from the yolk. When an egg is cooked for too long or at a high temperature, sulfur-containing proteins in the white can release hydrogen sulfide. As the gas moves toward the yolk, it reacts with the iron there and forms iron sulfide, which creates the familiar green-gray color around the yolk.

So, is the egg safe to eat? Yes. The discoloration itself is harmless and does not mean the egg is spoiled. The flavor and texture may be slightly affected if the egg has been overcooked, but the green ring is not a sign that you need to throw it away.

The good news is that it’s easy to prevent. Once the eggs are fully cooked, transfer them immediately to cold or ice water and let them cool quickly. Avoiding excessive cooking time also helps keep the yolks bright yellow and prevents that gray-green ring from forming.

It may look unusual, but there’s nothing mysterious or dangerous about it. It’s simply food chemistry happening right in your kitchen—and once you understand the science, that strange green ring becomes much less alarming.