Agate, artificially coloured. Close-up of the cut and polished surface of an agate geode. Agate is a form of chalcedony, a cryptocrystalline variety of quartz. The bands are produced by the precipitation of crystals from solutions of mineral salts. The bands are often highly coloured due to the presence of impurities. In this case, however, the bands were colourless, and the blue was added using a dye bath.