Geodes are essentially rock cavities with crystals inside. They occur when gas bubbles become trapped in molten rock or rounded cavities form in sedimentary rock. Minerals dissolved in water seep into the space in the rock and are slowly deposited on the inside surface over time. Here, crystals form from a supersaturated solution and grow directly on the Alum attached to the eggshell by the glue. The egg dye gives the crystals their color and the result is very similar to a real geode.