Typically, geodes form in a spherical cap-shape, sometimes elongated vertically. The outer layer of the geode is made of celadonite, a dark green mineral in the mica family. This outer layer is very useful to miners looking for geodes, as the dark green celadonite is easily recognizable. Under the celadonite there is a layer of agate (chalcedony), then colourless quartz, then finally amethystine quartz. At Ametista do Sul, the geodes occur in a 40 to 50m thick basaltic lava flow that formed in the Lower Cretaceous period. When the lava formed, there were especially large gas bubbles trapped in the rock. Water entered the empty spaces through fractures, over time forming the amethyst geodes.