Thundereggs are a unique agate formation that occurs in the volcanic strata in the Pacific Northwest, USA. Bubbles that formed in pyroclastic volcanic flows millions of years ago have been transformed into these unique agate nodules. They get their name from the Native American tribes of the area who tell the story of how the volcanos in the Cascade mountain range would war with one another, stealing the eggs from the thunderbird and hurling them great distances at each other.
They represent a cornucopia of unique and beautiful agate formations that are used in everything from jewelry to spheres. Each is unique and different, though each known bed of eggs follows some sort of predictable theme. Thundereggs are the official state rock in Oregon and are a favourite among collectors all over the world.
(Thanks to Sticks-in-Stones Lapidary for that description)
This very pretty "thunder egg" came from the pink Esterel hills behind the Cote D'Azur. It contains red jasper, blue chalcedony and druzy quartz.
Very indepth "The Formation of Thundereggs"