Description
A thunderegg (or thunder egg) is a button-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 – although they can range from less than a centimeter in size to over a meter in size. They typically contain centers of chalcedony that may be fractured, followed by deposition of agate, jasper, or opal, either uniquely or in combination. Also common are quartz and selenite crystals, as well as various other mineral growths and inclusions. Thundereggs usually look like ordinary stones on the outside, but cutting them in half and polishing them can reveal intricate patterns and colors. A defining characteristic of thundereggs is that (like other agates) the individual beds they come from can vary in appearance, although they may maintain a certain identity within themselves. Thunderegg is not synonymous with either geode or agate. A geode is a simple term for a rock with a cavity in it, often with crystal formation/growth. A thunderegg on the other hand is a specific geological structure. A thunderegg can be referred to as a geode if it has a cavity in it (see illustration of Gehlberg specimen), but not all geodes are thundereggs, as there are many different ways for a cavity to form. Also, a thunderegg is just one of the forms that agate can take.
