sedimentary

Banded iron formation (ironstone)

Banded iron formation, chiefly hematite/magnetite with chert (SiO₂)

AI-generated identification · may be incorrect

Hardness about 5–6.5; dark gray to metallic-black iron-rich bands with red-brown hematite, earthy to metallic luster, and very fine-grained cryptocrystalline structure. The specimen appears massive and weathered, with subtle red-gray banding rather than clearly exposed layers.

Identified August 30, 2026More sedimentary →

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Physical properties

Formation & geological history

Chemical sedimentary rock precipitated from iron- and silica-rich Precambrian oceans, mainly 2.5–1.8 billion years ago, when rising oxygen levels oxidized dissolved iron. It was later compacted, silicified, and commonly metamorphosed.

Uses & applications

Iron ore where hematite or magnetite is sufficiently concentrated; otherwise used as landscaping stone, lapidary material, or a mineral-collection specimen.

Geological facts

Red coloration typically reflects hematite, while darker portions may contain magnetite or iron-rich silicates. A magnet can help detect magnetite, but not all BIF is strongly magnetic.

Field identification & locations

Common in ancient shield terrains, especially the Lake Superior region, Western Australia, Brazil, India, and South Africa. Identify by dense weight, rusty-red streaking, and alternating iron-rich and silica-rich layers; a streak test and magnet are useful.