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Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. High Quality Content by WIKIPEDIA articles! A thunderegg (or thunder egg) is a nodule-like geological structure, similar to a geode, that is formed within a rhyolitic lava flow. 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. They usually contain centres of chalcedony in the form of agate, jasper or opal, either uniquely or in combination. Also frequently encountered are quartz and selenite crystals, as well as various other mineral growths and inclusions. Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colors. A characteristic feature of thundereggs is that (like other agates) the individual beds they come from can vary in appearance, though they can maintain a certain specific identity within them.

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