Before every rockhounding trip, we do our homework. We research locations, study maps, save coordinates, check access, and make lists of what we're supposed to find when we get there. By the time we leave home, our maps are covered with little pins, and we have enough research to convince ourselves that we know exactly what we're doing. Then we get there and discover that apparently the rocks had changed the assignment. Our August trip through Montana, Wyoming, and South Dako

From the outside, a geode often looks like an ordinary rock. Crack it open, however, and you may discover a glittering crystal-lined chamber that has been forming for millions of years. But why are geodes hollow in the first place? Learn how these fascinating formations develop, why crystals grow inside them, and what determines whether they remain hollow or become completely mineral-filled.