A thunderegg (or thunder egg) is a nodule -like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers. Thundereggs are rough spheres, most about the size of a baseball —though they can range from a little more than a centimeter (one half inch) to over a meter (three feet) across.

A Thunder Egg “Thunderegg” is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers. Thundereggs are rough spheres, most about the size of a baseball—though they can range from less than an inch to over a meter across.