Limonite. Image source: Mineral Information Institute.
Limonite, also called brown iron or bog ore, is a generally dark brown, amorphous oxide mineral consisting of hydrated iron (III) oxide (FeO[OH].nH2O). A major iron ore of widespread occurrence, often with goethite, it is formed by alteration of other iron minerals. Limonite may appear lustrous black, brown to yellow, and is streaky, porous, and often mixed with sand and clay.