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, allowing crystals to grow. Common minerals found in geodes include quartz, calcite, and amethyst. Geodes can vary in size, from small, golf ball-sized specimens to large formations that can weigh hundreds of pounds. When cut open, they reveal stunning interiors filled with sparkling crystals, making them popular among collectors and in decorative arts. Their unique beauty and geological significance make geodes fascinating natural wonders.