Hematite, chemically identified as iron(III) oxide (Fe2O3), is one of Earth's most abundant minerals, playing a critical role in geology and industry. Its formation is a complex process influenced by various geological environments, leading to its widespread presence in sedimentary, metamorphic, and igneous rocks. Understanding these formation mechanisms and identifying characteristics is vital for geologists, miners, and material scientists alike.

One of the most significant ways hematite forms is through the weathering of iron-bearing minerals. Surface exposure to oxygen and water oxidizes iron compounds, transforming them into hematite. This process is a primary reason for the characteristic reddish-brown color found in many soils and rock formations. Another crucial formation pathway is sedimentation. Iron-rich sediments, deposited in ancient aquatic environments, undergo geological transformations over eons to become ironstone deposits, with hematite being a key constituent. These sedimentary deposits, often found in extensive layers, are the world's richest sources of iron ore.

Metamorphism also contributes to hematite's occurrence. Under intense heat and pressure within the Earth's crust, iron-bearing rocks can recrystallize, forming hematite-rich metamorphic rocks like schist. Hydrothermal activity, involving the circulation of hot, mineral-laden fluids, can also lead to hematite precipitation in veins and crystal formations. The diverse geological origins of hematite mean it can be found in a wide array of settings, from ancient seabed deposits to volcanic regions.

Identifying hematite is often made easier by its distinctive physical properties. While its color can range from metallic gray to earthy red, and its luster from dull to metallic, its streak is consistently reddish-brown. This hematite streak test, achieved by rubbing the mineral against an unglazed porcelain surface, is a reliable diagnostic tool. Furthermore, hematite is moderately hard (5.5-6.5 on the Mohs scale) but brittle. Unlike magnetite, pure hematite is not magnetic, though some specimens may exhibit weak magnetism due to impurities.

The study of hematite's geology of iron deposits and formation processes provides critical insights into Earth's history and the availability of essential industrial raw materials. Its presence on Mars, identified by space probes, further highlights its significance in understanding planetary geology and the potential for past or present life.