Sedimentary Formation: The Banded Iron Formations
The most massive hematite deposits on Earth formed between 2. 5 and 1. 8 billion years ago, during the Great Oxidation Event. Before photosynthetic cyanobacteria evolved, Earth's oceans were saturated with dissolved iron. When oxygen appeared in the atmosphere for the first time, it reacted with that dissolved iron, precipitating vast sheets of iron oxide that settled to the ocean floor in alternating layers with silica.
These are the banded iron formations (BIFs), the source of most commercial hematite mined today in Australia, Brazil, and the Lake Superior region.
The BIFs tell a story older than complex life itself. Every piece of banded hematite you hold records the moment Earth's atmosphere changed forever, the transition from an anoxic world to one that could eventually support everything that breathes.
Hydrothermal Formation
Hematite also crystallizes from iron-rich hydrothermal fluids moving through volcanic rock. As superheated water carrying dissolved iron encounters cooler rock or changes in pressure, the iron precipitates as hematite crystals. This process creates specular hematite, the brilliant, mirror-like variety with visible crystal faces that reflect light like polished steel. The "iron rose" formation, where thin tabular crystals stack in rosette patterns, also forms through slow hydrothermal crystallization in Alpine fissures and volcanic cavities.
Weathering and Oxidation
The simplest formation pathway: any iron-bearing mineral exposed to oxygen and water will eventually oxidize to hematite. This is why Mars is red, its surface is coated in hematite formed from the oxidation of iron-rich basalt. On Earth, this same process creates the red soils of the American Southwest, the Australian Outback, and laterite deposits across the tropics. Red ochre, the powdered hematite used by humans for at least 100,000 years, is this weathered form.
What makes hematite mineralogically unique is its streak. Despite appearing metallic silver-grey or black on the surface, scratch hematite across unglazed ceramic and it leaves a blood-red mark. This is why the Greeks named it haima lithos, blood stone. The streak never lies: if it's red, it's hematite, regardless of what the surface looks like.
Mineralogical data: Dana, J.D. & Hurlbut, C.S. (1971). Manual of Mineralogy. Wiley. | Klein, C. & Dutrow, B. (2007). Manual of Mineral Science. Wiley.