Thundereggs are round, solid nodules that form in volcanic ash layers and are often filled with beautiful minerals like agate, jasper, or quartz. Unlike geodes, which are usually hollow, thundereggs are typically solid all the way through and must be cut open to reveal their hidden beauty. They form when mineral-rich water seeps into gas pockets in cooling lava, gradually filling them with layers of minerals over time. Each thunderegg is unique, with colorful patterns, swirls, or even crystal-lined cavities inside. These geological gems are especially famous in Oregon, where they’re the official state rock!