(c) 2002 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Thunder eggs are a kind of solid geode: a rocky shell with a filling of agate, the refined variety of the mineral chalcedony. They form in rhyolite lava beds, perhaps where the volcanic gases left bubbles frozen into the rock. Later, mineral solutions deposited the filling. Thunder eggs are the state rock of the state of Oregon. More precisely, they should be called the state stone since thunder eggs aren't a rock type.