These are geode rocks with gorgeous druzy crystals inside. You can crack them open yourself, the best method is to use a hammer and chisel. It's suggested that you use a flat-faced chisel and not the pointed type, because the force from a pointed chisel will be directed over one spot versus over a larger area. Continue this process until you see a crack develop in the geode, and then follow the crack around the geode until it opens. Begin lightly at first and then strike harder if a crack is not developing.