Can you please explain why a brown "free range" chicken egg that I hard-boiled had a blue-green colored "egg-white".  I cut it in half and it was blue-green all the way through to the yolk, which was golden, but tinged with blue-green around the edge. I ate half of this egg before it dawned on me that this might not be just some unusual feature of "free range" eggs.

Can you also telling me if floating an egg in water is an accurate test of whether it is okay to eat it.  I have been told that if it floats it is bad; sinks means it is okay. - Chris (7/19/05)

The harmless greenish ring is due to an iron and sulfur compound which forms when eggs are overcooked or not cooled quickly.

Green ring around yolk:  Caused by iron of the yolk combining with sulfur of the white.  It is due to overcooking.  Stopping cooking by rinsing in cold water or the ice treatment usually prevents the ring. 

The floating technique is used to tell raw eggs from boiled eggs. Read about it on this page:  http://whatscookingamerica.net/eggs.htm