Why Does the Green Color Appear Around the Egg Yolk?

Ever noticed a grayish-green halo around your hard-boiled egg yolks? It might look a bit odd, but it’s not only harmless—it’s pure science at work.

Sulfur + Iron = Iron Sulfide When eggs are cooked too long or at high temperatures, the sulfur in the egg white reacts with the iron in the yolk, forming iron sulfide, which gives the yolk that greenish-gray ring.
It’s All About the Heat The hotter and longer the egg cooks, the more hydrogen sulfide builds up and travels to the yolk, increasing the intensity of the ring.