Why does the yolk of an egg turn green and how can I prevent it?

No worries about eating them, they're just cooked long enough to have developed the sulfur.

Next time boil the eggs hard for 9 minutes. Drain, and plunge into a large bowl filled with cold water and ice. Drain and peel or pop them in the refrigerator for an hour.

There is an extremely important step which prevents the greenish “ring” from forming on the surface of the yolk.

To prevent this and other problems,

see "The Classic Hard Boiled Egg"

at The Good Egg (link provided below).

You are overcooking your eggs..well 1/2 of them. I boil my eggs then turn off the heat and let them sit in the water for 8 minutes. I would try that. I used to have the same problem.

This is alright, it is natural for it to happen like that. When I saw the question I was thinking, "Did he hard boil them?" Becuase the yolk tends to turn green when you do.

they are fine to eat...you just cooked them too long....start with cold water and put in the eggs...bring them to a boil and then cover the pan, shut off the burner and let them sit for 10-12 minutes....run under cold water....they should be perfect

It is usually caused by overcooking them. Try boiling them for a little less time.

add anything citrus to the water