Hematite is the mineral form of iron (111) oxide (Fe2O3), one of several iron oxides. It is the oldest known iron oxide mineral and is widespread in rocks and soils.
Hematite is colored black to steel or silver-gray, brown to reddish brown, or red. Gray hematite is typically found in places that can have still standing water or mineral hot springs, such as those in Yellowstone National Park in North America.
The mineral can precipitate out of water and collect in layers at the bottom of a lake, spring, or other standing water. Hematite can also occur without water, however, usually as the result of volcanic activity.
Clay-sized hematite crystals can also occur as a secondary mineral formed by weathering processes in soil, and along with other iron oxides or oxyhydroxides such as goethite, is responsible for the red color of many tropical, ancient, or otherwise highly weathered soils.