What does it mean when a hard boiled egg yolk is GREY?

Chef's answer
A greenish-gray ring may appear around a hard-cooked egg yolk. It's unattractive, but not harmful. The ring is caused by a chemical reaction involving sulfur (from the egg white) and iron (from the egg yolk), which naturally react to form ferrous sulfide at the surface of the yolk..
Frequently asked Questions 🎓
Egg It On. Although I prefer my burgers without it, for a richer, more cohesive patty, add one egg yolk per pound of meat.
A greenish-gray ring may appear around a hard-cooked egg yolk. It's unattractive, but not harmful. The ring is caused by a chemical reaction involving sulfur (from the egg white) and iron (from the egg yolk), which naturally react to form ferrous sulfide at the surface of the yolk..
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