Agate. Slice through a nodule of agate, a type of chalcedony. This is a cryptocrystalline variety of the mineral quartz (silicon oxide). At the heart of the agate are rock crystals, a clear, colourless form of quartz. Impurities within agate can give the mineral a wide range of colours distributed in concentric bands. The bands represent successive layers of quartz precipitated out of solution. This occurs inside the circular cavities of lava rocks, producing rounded nodules. Fine specimens of agate are found in Brazil, Italy, Uruguay and Germany.