Jasper is a common variety of chalcedony containing admixed hematite or goethite. It is normally red to brown, sometimes yellow or gray to green, often with banding and spotting. Good grades are used for semiprecious gemstones. Jasper is found in Greece, Siberia, Libya, and the River Nile valley. It was once believed that stomach complaints could be cured by wearing these stones. A dark form of jasper was traditionally used as a touchstone to estimate the gold content of precious alloys.