Jasperoid is a rare, peculiar type of metasomatic alteration and occurs in two main forms; sulfidic jasperoids and hematitic jasperoids. True jasperoids are different from jaspillite, which is a form of metamorphosed chemical sedimentary rock, and from jasper which is a chemical sediment.
Sulfidic jasperoids are typical examples of silica-sulfide metasomatism of dolomites, and are found in Nevada, Australia and Iran. They are hard, dense purple-black rocks with considerable content of pyrite. The bodies in Nevada are quite thin (seldom greater than 8 m) and stratabound.