"A thunderegg is a type of rock similar to a geode but formed in a rhyolitic lava flow and found only in areas of volcanic activity. Thundereggs are rough spheres, most about a big as a baseball. They look uninteresting on the outside, but slicing them in half may reveal highly attractive patterns and colors valuable in jewelry."
Um.
not sure on thundereggs but I know Geodes are NOT recommended because Geodes primarily consist of limestone, however many are quartz based. Sometimes minerals contained within the hollow are harmful & even those that are safe? are often razor sharp and can be lethal to fish.