Hematite

Hematite is the most important iron ore on Earth and one of the most abundant minerals in the crust. It forms across a staggering range of environments - from volcanic fumaroles to ancient ocean floors to the surface of Mars (hematite is what makes Mars red).

The most geologically significant hematite deposits are banded iron formations (BIFs) - layered sedimentary rocks laid down 2-3 billion years ago when Earth's atmosphere had almost no oxygen. Iron dissolved in ancient oceans was oxidized by the first photosynthetic organisms, precipitating as layers of hematite alternating with silica. These deposits, found in Australia, Brazil, South Africa, and the American Iron Range, contain the vast majority of the world's iron ore.

Specimen-quality hematite takes many forms: specular hematite (flat, mirror-like crystals), kidney ore (smooth, rounded botryoidal masses), iron roses (thin, overlapping tabular crystals arranged like petals), and massive earthy hematite used as red pigment since prehistory.

Hematite's most diagnostic feature is its red-brown streak - regardless of whether the specimen appears metallic silver, steel gray, or earthy red, the streak is always red-brown. This is the test. No other common metallic mineral has a red streak.

At hardness 5.5, hematite can be scratched by a steel file but not by a copper coin. Its high specific gravity (5.26) gives it a distinctive heft - hematite feels noticeably heavy for its size. Metallic varieties show a mirror-like luster on fresh surfaces. Magnetic hematite (sold as 'magnetic hematite' in crystal shops) is usually synthetic ceramic, not genuine hematite - real hematite is not magnetic.