Agate forms within volcanic and metamorphic rocks. Over time, silica-rich water seeps into cavities and slowly deposits layers of chalcedony, a microcrystalline form of quartz. These layers build up, often in concentric rings, creating the characteristic banding patterns. The process can take millions of years.The distinct bands in agate result from variations in mineral content and impurities during formation. These impurities can include iron, manganese, and other minerals that contribute to the wide range of colors seen in agate slices.Agates often form in voids or cavities within rocks. When sliced, these cavities sometimes appear as central voids or pockets within the agate, adding to the uniqueness of each piece.