Why is a dark ring sometimes seen around the yolk of a boiled egg?

Most people over cook their hard boiled eggs. Hydrogen sulphide generated by the decomposition of sulphur-containing amino acids in the white will react with iron in the yolk causing a (harmless) grey-green film of ferrous sulphide to form on the surface of the yolk.

Put 2 week old eggs (fresh eggs are hard to peel, and by the time eggs reach the store they are about a week old) in saucepan covered by a inch of water.

Bring to a rolling boil on high.

Immediatly reduce heat to medium and leave for 10 minutes.

Put eggs into ice water for a few minutes.

Peel.