A geode is a sphere shaped rock which contains a hollow cavity lined with crystals. A geode which is completely filled with small compact crystal formations such as Agate, Jasper, or Chalcedony is called a nodule. The only difference between a geode and nodule is that a geode has a hollow cavity, and a nodule is solid. We carry everything from very small Ocos geodes to large cathedrals of Amethyst and Citrine. And Agate slices and slabs.