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

Signs of decay in other foods

Combined with sulfur smell, it led to the misconception that:

The egg had gone bad

In reality:

Spoiled eggs show green, pink, or black discoloration throughout the white

Rotten eggs smell strongly before cooking

The green ring only appears after cooking

19. The chemistry in one sentence

The green ring forms because overheating causes sulfur from egg whites to react with iron in the yolk, creating greenish iron sulfide at their boundary.

20. Conclusion

The green ring around a hard-boiled egg yolk is a classic example of simple chemistry at work in everyday life. It forms when heat breaks down sulfur-containing proteins in the egg white, releasing hydrogen sulfide gas that migrates toward the iron-rich yolk. There, the sulfur and iron react to form iron sulfide, which appears as a green or gray-green halo. The phenomenon is encouraged by high temperatures, long cooking times, and older eggs, but it is completely harmless and does not indicate spoilage.

In short, the green ring isn’t a warning sign—it’s a chemical footprint of overcooking. Understanding it not only demystifies a common kitchen occurrence but also highlights how fundamental chemical principles quietly shape the foods we eat every day.