Agate. Slice through a nodule of agate, a type of chalcedony. This is a cryptocrystalline variety of the mineral quartz (silicon oxide). Impurities within agate can give the mineral a wide range of colours. The outer banding represents successive layers of quartz precipitating out of solution. This occurs inside the circular cavities of lava rocks, producing rounded nodules. Fine specimens of agate are found in Brazil, Italy and Germany.