Geodes are prized by rock collectors because of their dazzling appearance. A geode is a rock that has a crystalline formation trapped inside; they make attractive display pieces. Some geodes are hollow on the inside, while others are made up of solid crystal.
Cut open the geode with a wet-saw. The crystalline structure of a geode is exposed only after the whole rock has been sawed in half.
Check if the geode is completely filled on the inside, or if it is has an open space with crystals. Open geodes are simply called "geodes." Those that are filled are known as "nodule" geodes.
Look at the shape of the geode. Geodes are identified by shape, as well as by the material they contain. Cathedral geodes are tall and skinny, while double-chambered geodes have two empty spaces inside a single half of the geode. Other types include cubic, barrel and rhombohedron geodes.
Look at the color and quality of the crystals inside your geode. Dark purple crystals indicate an amethyst geode. These are more rare than quartz geodes. Quartz geodes might be white or red inside. Geodes not filled with quartz or amethyst crystals are typically filled with calcite. Black calcite is one of the rarest types of geode.
Compare your geode to a reference website or book. Geode identification is primarily visual.