Geodes are hollow, rock-like formations that contain crystals or mineral matter inside. They typically form in volcanic or sedimentary rocks when mineral-rich water fills a cavity and slowly deposits minerals over time. The outer shell is often rough and unassuming, while the interior can reveal stunning crystal structures, such as quartz or amethyst. These natural wonders can vary greatly in size, from small pebble-sized geodes to large formations several feet across. Geodes are often collected for their beauty and are popular among mineral enthusiasts and collectors. They can also be cut open to reveal their intricate interiors, making them fascinating specimens for display.