geode is a round rock which contains a hollow cavity lined with crystals. Rocks which are completely filled with small compact crystal formations such as agate, jasper or chalcedony are called nodules. The only difference between a geode and a nodule is that a geode has a hollow cavity, and a nodule is solid. Depending on the type of geode, they are rated between 6.5 and 7.5 on the Mohs hardness scale. Geodes are thought to improve intellectual insight, awareness and community spirit, as well as help to attract compatible friends and lovers.