Watch this short video on breaking open a geode. Before filming, I gave the geode a few hits with a hammer and screwdriver until a crack started to form. Before having it crack any more, I started to film. As you see, a small hit caused the geode to split in two. Unfortunately, the shiny crystals in this geode are quite small, but it’s a fun activity for kids and adults alike.
As stated in Wikipedia, geodes or more or less rounded formations in volcanic (igneous) and sedimentary rocks. Most of the what is inside a geode is quartz or calcite. While there isn’t an exact understanding of how geodes formed,the basic understanding is that the geodes formed from bubbles in volcanic rock or sedimentary, while the crystals formed later, with the slow seeping of mineral-laden water into the bubble.