The green ring around the yolk in an improperly cooked and/or cooled hard-cooked egg is ferrous sulfide.

Name: The green ring around the yolk in an improperly cooked and/or cooled hard-cooked egg is ferrous sulfide.
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Ferrous sulfide is a compound that forms when iron from the egg yolk reacts with hydrogen sulfide from the egg white in an environment lacking oxygen, such as when the egg is overcooked or not cooled quickly enough. This reaction causes the green ring to form around the yolk.

The Green Ring Around the Yolk in an Improperly Cooked