Bookends, Crosscut Agate Rock Crystal Thunderegg. Mineral Specimen. Size (together): 4 1/2 X 3 1/2X 5 1/2 in. #5343 . A thunderegg is a rock similar to a geode that can form within rhyolitic volcanic ash layers when lava and water combine. They are rough spheres about the size of a baseball, though some have measured almost a meter across, and usually look like ordinary rocks on the outside. Slicing them in half and polishing them reveals intricate patterns and colors, and the center is composed of chalcedony that appears like a filmy hardened liquid due to silica blending with the ash. 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. They are found globally where conditions are optimal, with Oregon in the US being the most famous location. On March 30, 1965, the thunderegg was designated as the Oregon state rock by a joint resolution of the Oregon Legislative Assembly. The world’s largest thunderegg is 1.75 tons and is housed by the Rice Northwest Museum of Rocks and Minerals in Hillsboro, Oregon. Native American legend says that the rocks are the eggs of the thunderbirds which supposedly occupied Mount Hood and Mount Jefferson. Thunder Spirits on the mountains hurled the “eggs” at each other for sport.