Due to the volcanic nature of the Watchungs, zeolites, including prehnite, analcime, and stilbite, which form from a reaction of mafic rocks in alkaline environments, can be found along exposed ridge lines. Agate, primarily in the form of chalcedony, and crystalline quartz (sometimes in the form of amethyst), are prominent in the ancient lava flows of the Watchungs and are typically seen as embedded nodules along exposed fronts. Datolite, another nodular mineral, has been found embedded in the volcanic rock around the Great Falls. Additionally, jasper and satin spar are known to exist within the northwestern Preakness Range.