That unsightly surface color is produced when extreme heat or prolonged cooking chemically combines the sulfur in the eggs with the iron in the yolk to form a harmless substance called ferrous sulfide. You can eliminate the problem by following three rules. First, use fresh eggs (ferrous sulfide develops more readily in older eggs). Second, never cook the eggs longer than 15 minutes. Third, once cooked, bring a halt to further cooking by quickly cooling the eggs in cold water.
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