Hematite is a rock-forming mineral found in sedimentary, metamorphic and igneous samples. Hematite is an iron oxide with the chemical formula Fe2O3 and the most important ore of iron. The largest deposits of hematite formed around 2.4 billion years ago when cyanobacteria began to photosynthesize in the iron-rich oceans, mixing oxygen and iron to which sunk to the bottom and formed large iron ore deposits and banded iron formations. Hematite today is usually found on the surface and in the shallow crust occurring as banded iron formations or in hydrothermal settings. Hematite's most diagnostic property is its red-brown streak seen in the image above. This mineral shows no cleavage, varies in colour from dark-grey to red-brown depending on the oxidation level of the mineral. Hematite also varies in lustre from metallic when unoxidized, to submetallic, to earthy/dull when oxidized like this sample. The most common uses of hematite are as a red pigment in paint materials and as radiation shielding for X-rays.