A geode is a hollow rock formation that contains mineral crystals inside. These formations often start as bubbles in volcanic rock or as cavities in sedimentary rock. Over time, mineral-rich water seeps into these cavities, depositing minerals that crystallize and create beautiful patterns and colors. Geodes can vary in size, from small ones that fit in your hand to large formations that can weigh hundreds of pounds. Common minerals found in geodes include quartz, amethyst, and calcite. They are often collected for their aesthetic appeal and are popular among rock hounds and collectors.