That green (or gray-green) ring around a hard-boiled egg yolk is completely harmless — and it’s caused by a simple chemical reaction.
🧪 What Causes It?
When eggs are overcooked or cooked at too high a temperature:
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The egg white releases hydrogen sulfide (a sulfur compound).
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The yolk contains iron.
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Sulfur + iron react to form iron sulfide.
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Iron sulfide appears as a greenish-gray ring around the yolk.
It’s just chemistry — not spoilage.
🕒 Why It Happens
The ring usually forms when:
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Eggs are boiled too long
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They cool too slowly
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They sit in hot water after cooking
The longer the yolk stays hot, the more reaction occurs.
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