Agates are a type of chalcedony, which is a cryptocrystalline quartz. Agates grow in the voids of rock masses such as lavas, building themselves in layers from the outside-in over millions of years. As the softer, surrounding rock erodes away, the agates remain. Sometimes these agates are themselves eroded and smoothed into pebbles in riverbeds. Other times they can be found as solid nodules that can be sliced and policed, or hollow geodes...