sedimentary

Banded iron formation fragment (polished ironstone)

Banded iron formation; hematite/magnetite-rich chert, commonly approximated as Fe₂O₃–Fe₃O₄ with SiO₂

AI-generated identification · may be incorrect

Dark gray-black and rusty brown, fine-grained, opaque, with a subdued metallic-to-earthy luster; hardness about 5.5–6.5 Mohs. The elongated, smoothly rounded surface appears polished or naturally water-worn, and iron-rich zones may be magnetic if magnetite is present.

Identified September 10, 2026More sedimentary →

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Physical properties

Formation & geological history

Chemical sediment deposited in ancient Precambrian oceans, where dissolved iron precipitated with silica; most major deposits formed roughly 3.8–1.8 billion years ago. Later lithification, metamorphism, and weathering produced the iron-oxide colors.

Uses & applications

Used chiefly as iron ore and as attractive polished decorative stone, paperweights, and lapidary pieces; not normally used as a gemstone.

Geological facts

Rusty coloration reflects oxidation of iron minerals, while black areas may contain magnetite or hematite. The specimen’s exact identity cannot be confirmed from an image alone; streak, magnetism, and a fresh surface are useful tests.

Field identification & locations

Common in regions such as the Mesabi Range, Western Australia, Brazil, India, and South Africa. Look for alternating silica-rich and iron-rich bands, high density, and a red-brown streak; estimated value is about $5–$30 for a small common polished specimen, occasionally $50–$100 for distinctive pieces.