"Hard-cooked eggs (eggs cooked in the shell in water) should never be boiled - simmer them in water. If boiled or cooked too long, the protein toughens or becomes rubbery and a greenish or purplish ring forms around the yolk.
To correctly cook the eggs, place them in a single layer in a pan with enough cold water to cover them completely. Bring the water to a boil, remove from heat, cover tightly with a lid, and allow to remain in the water approximately 15 to 20 minutes. Then place under running, cold water to cool quickly. This way of cooking is also known as 'coddling.' It does not toughen the whites as boiling does."
Boiling Eggs - soft-cooked, medium-cooked, hard-cooked
