Thunder eggs are fascinating geological formations that typically form within volcanic rocks. They are essentially nodules or geodes that contain agate, chalcedony, or other minerals. These nodules form in the gas pockets or cavities within volcanic rock, often in rhyolite or similar types of lava flows.
The following cabochons where made from thundereggs.
Del Norte Thunderegg Cabochon
A thunderegg (or thunder egg) is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers.[1] 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. They usually contain centres of chalcedony which may have been fractured followed by deposition of agate, jasper or opal,[1] either uniquely or in combination. Also frequently encountered are quartz and gypsum 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 colours. 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.
Thunderegg is not synonymous with either geode or agate. A geode is a simple term for a rock with a hollow in it, often with crystal formation/growth. A thunderegg on the other hand is a specific geological structure. A thunderegg may be referred to as a geode if it has a hollow in it, but not all geodes are thundereggs because there are many different ways for a hollow to form. Similarly, a thunderegg is just one of the forms that agate can assume.
Thundereggs are found globally where conditions are optimal. In the US, Oregon is one of the most famous thunderegg locations. Germany is also an important center for thunderegg agates (especially sites like St Egidien and Gehlberg). Other places known for thundereggs include Ethiopia,[2] Poland,[3] Romania, Turkey, Mexico, Argentina,[4] Canada, Mount Hay[5] and Tamborine Mountain (Australia),[6] and the Esterel massif (France).[7]
