A greenish-gray ring around the yolk of a hard-boiled egg may look concerning, but it does not mean the egg is spoiled or unsafe. It usually appears when the egg has been cooked too long or remained hot after cooking.
The discoloration forms through a natural chemical reaction. Heat releases sulfur compounds from the egg white, which react with iron in the yolk and produce a thin layer of iron sulfide. The result is the familiar green or gray border.
Although completely harmless, the ring often accompanies a dry, chalky yolk and rubbery egg white. Fortunately, proper timing and rapid cooling can prevent it.
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