Limonite mineral. This mineral consists of forms of hydrated iron oxide and iron hydroxides. It is one of the ores mined as a source of iron. It is naturally yellowish brown, but the polished surface seen here is black. The botryoidal (kidney ore) form seen here is one of the more common shapes taken by this mineral. Limonite is also known as the lemon rock, and has been used to produce the pigment ochre. This sample, from the Siegerland of Westphalia, Germany, is 160 millimetres in height.