sedimentary

Banded iron formation fragment (tumbled iron-rich rock)

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

AI-generated identification · may be incorrect

Typically 5.5–6.5 Mohs; dark metallic gray to rusty brown, earthy to submetallic luster, very fine-grained, dense, and commonly weakly magnetic if magnetite-rich. The rounded shape indicates natural water wear or tumbling/polishing.

Identified September 4, 2026More sedimentary →

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Physical properties

Formation & geological history

Chemical sediment deposited in ancient oceans, where dissolved iron precipitated with silica during oxygenation events, mainly about 3.8–1.8 billion years ago. Major deposits occur in Precambrian shields.

Uses & applications

Used as iron ore when sufficiently rich; attractive pieces are sold as educational or lapidary specimens, though this example appears mainly collectible rather than gem-grade.

Geological facts

Rusty brown areas are oxidized iron minerals such as goethite or limonite; black areas may contain magnetite or hematite. Alternating iron- and silica-rich bands are the diagnostic feature, though they are subdued here.

Field identification & locations

Common sources include the Lake Superior region, Labrador, Western Australia, Brazil, India, and South Africa. Check for magnetic response, unusually high density, and thin gray/red banding; a streak test may show red-brown hematite or black magnetite.