A thunderegg is a geode filled with agate or common opal. It began as a gas bubble in a lava flow. Water entered the bubble, filling it with silica. Thundereggs were named by the Warm Springs Indians of Oregon. They believed that spirits robbed the Thunderbird’s nest and threw the eggs about. They are usually found in volcanic ash and are up to 60 million years old. The thunderegg is the state rock of Oregon.