Geodes can occur in both igneous and sedimentary rocks. Bubbles in lava can form cavities in igneous rocks as they cool, while animal burrows, desiccated tree roots or mud deposits in sedimentary rocks may create similar hollows. When mineral-rich water flows through these voids, it can leave behind deposits that eventually form the crystal structure of a geode.
While quartz crystals are common inside geodes, many different types of crystals can form depending on the mineral content of the water that passed through the structure. Agate, amethyst, chalcedony and jasper are frequently found in geodes, as are calcite, dolomite and celestite crystals. Typically, the interior surface of the hollow fills entirely with crystalline growths, creating a closed, rounded structure that eventually breaks free of the surrounding rock and reaches the surface due to seismic activity. The colors inside a geode may be uniform, or there may be multiple colored bands, much like rings on a tree, indicating the passage of different chemical compounds through the structure.
Geodes can vary greatly in size. Small geodes that can fit in the hand are common decorative items, but even large voids may fill with quartz and other crystals to form enormous geodes. As of 2016, the largest known geode is the Crystal Cave in Ohio, a celestite cavern measuring 35 feet across at its widest point.