From the outside, Geodes appear to be ordinary concretions of sedimentary rock. These nodules were hollow at one time, and crystals have begun to fill in the void.  Some Geodes fill in completely with mineral while others are still hollow.  If you saw or break open a Geode, you will find a treasure of agate, minerals, and crystals, resembling a miniature cave.  Wrapping it in an old cloth or sock and tapping it with a hammer until the Geode cracks open can break a whole, uncut Geode.