sedimentary

Banded iron formation (ironstone), possibly jasper–hematite

Banded iron formation; chiefly hematite (Fe₂O₃), magnetite (Fe₃O₄), and silica (microcrystalline quartz/chert)

AI-generated identification · may be incorrect

Dark gray to black, fine-grained and massive, with rusty orange weathering zones from oxidized iron minerals; metallic to dull luster, no obvious cleavage, hardness roughly 5–6.5, and commonly strongly magnetic if magnetite-rich.

Identified August 30, 2026More sedimentary →

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Physical properties

Formation & geological history

Chemical sediment formed when dissolved iron precipitated with silica in ancient oceans, most commonly during the Precambrian (about 3.8–1.8 billion years ago), then compacted and lithified.

Uses & applications

Major global source of iron ore for steelmaking; attractive polished pieces are sold as lapidary or educational specimens, though this rough fragment has modest commercial value.

Geological facts

Its red-orange surfaces reflect hematite or limonite produced by oxidation, while the dark portion may contain magnetite or iron-rich chert.

Field identification & locations

Identify it by very fine layering or banding, high density, rusty streak, and a magnet response; common sources include Australia, Brazil, India, South Africa, and Michigan’s Lake Superior region. Estimated value: $2–$15 for a small rough piece, or $20–$100 if strongly banded and polished.