Geodes are rocks that have been naturally hollowed out and lined with crystals. They are formed when gas bubbles in molten rock become trapped in the cooling lava, and as the lava cools and hardens, the gas bubbles form hollow cavities. The cavities are then filled with mineral-rich water, which slowly evaporates and leaves behind a crystal-lined interior. Geodes are often found in sedimentary and volcanic rocks, and can range in size from a few centimeters to several meters in diameter.