Hematite is a hard mineral consisting of iron (III) oxide (αFe2O3); it is the chief ore of iron. Hematite crystallizes in the rhombohedral system, with the corundum structure. It comes in two forms: red and gray. Red hematite is essentially rust; as well as a source of iron, it is used in paints (ochre) and polishes (rouge). Common on Earth, it also occurs everywhere on Mars and is responsible for the planet's distinctive red color.
Gray hematite has a dark gray metallic luster. Unlike the red variety, it usually forms over long periods of time, in the presence of liquid water, often standing bodies of liquid water. Gray hematite has been found on Mars in only three places, Meridiani Planum (the landing site for one of the Mars Exploration Rovers, Aram Chaos, and Valles Marineris. It was first identified on Mars by the Thermal Emission Spectrometer (TES), aboard Mars Global Surveyor.