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

That green ring around a hard-boiled egg yolk is common and harmless, but it usually means the eggs were overcooked. It happens when iron in the yolk reacts with sulfur in the white, creating a thin layer of iron sulfide. The fix is simple: start by placing eggs in a pot in a single layer, cover them with cold water by a few centimeters, then heat gently and avoid prolonged boiling. Careful timing and quick cooling after cooking help keep the yolk bright, tender, and appetizing without any chef-level skill required.