sedimentary

Iron-rich sedimentary rock, likely banded iron formation fragment

Banded iron formation (BIF), composed chiefly of hematite/magnetite and chert

AI-generated identification · may be incorrect

Dark brown to black, rusty metallic surfaces with reddish-brown weathering; likely Mohs hardness 5–6.5, submetallic to earthy luster, dense, and commonly fine-grained with possible layered silica bands; magnetism depends on magnetite content.

Identified September 13, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from chemical precipitation of iron oxides and silica in ancient oxygenated oceans, chiefly during the Precambrian, about 3.8–1.8 billion years ago. The image may instead show an iron-rich concretion or weathered ironstone, which cannot be confirmed visually.

Uses & applications

BIF is an important iron ore source; attractive pieces are used as cabinet specimens, lapidary material, and occasionally ornament. These rough fragments have little commercial value unless unusually banded or mineral-rich.

Geological facts

Rusty coloration commonly reflects limonite/goethite formed by weathering of hematite or magnetite. A magnet test can help detect magnetite-rich material.

Field identification & locations

Look for alternating iron-oxide and pale chert layers, high density, and possible magnetism; common localities include Minnesota, Michigan, Western Australia, Brazil, India, and South Africa. Estimated value: $2–$20 for ordinary fragments, up to $50–$150 for attractive polished specimens.