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What causes the greenish gray color around... | Almanac.com

What causes the greenish gray color around...

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What causes the greenish gray color around the yolks of hard-boiled eggs?
This is a harmless compound of iron and sulfur called ferrous sulfide, which forms when an egg is heated. To prevent its formation, boil the egg only as long as is necessary to set the yolk, then plunge it into cold water and peel it promptly.

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