Thundereggs are fascinating, naturally occurring geologic formations typically found within rhyolitic volcanic ash beds. They are not meteorites or extraterrestrial objects, as their name might suggest, but rather a type of nodule or concretion. Thundereggs form in silica-rich volcanic rocks, particularly rhyolite. During volcanic eruptions, molten lava or volcanic ash is deposited, often containing gas bubbles or cavities known as vesicles. Over millions of years, groundwater containing dissolved silica and other minerals flows through the volcanic rock.This mineral-rich water deposits silica (usually in the form of chalcedony, agate, or quartz) into the cavities.The minerals gradually crystallize within the cavities, forming intricate patterns and colors. Different minerals may contribute to the vivid hues of the agate or quartz, including reds, blues, and whites.