Iron ores are rocks and minerals from which metallic iron can be extracted. The ores are usually rich in iron oxides and vary in colour from dark grey to rusty red. The iron itself is usually found in the form of magnetite (Fe3O4), hematite (Fe2O3), limonite or siderite. Hematite is also known as "natural ore". The name refers to the early years of mining, when certain hematite ores contained 66% iron and could be fed directly into steel-making blast furnaces. Iron ore is the raw material used to make pig iron, which is one of the main raw materials to make steel. 98% of the mined iron ore is used to make steel. Iron ore cargoes may affect magnetic compasses.