Magnetite may be the most important ore of iron. It is found at Kiruna, Sweden, and in other regions of ancient and metamorphic rocks. It (rarely) forms beautiful shiny black octahedral crystals, as well as rhombododecahedrons. It is hard, 5.5-6/5 and rather heavy, d = 5.18 g/cc. It dissolves in hydrochloric acid, and, of course, is strongly magnetic.