metamorphic

Banded iron formation (BIF) fragment

Metamorphosed iron formation; alternating hematite/magnetite and chert (SiO₂) layers

AI-generated identification · may be incorrect

Dark metallic-gray to rusty-brown, strongly banded, with white-to-tan microcrystalline quartz/chert. Hardness is about 5–6.5 (quartz and iron oxides), metallic to earthy luster, no prominent cleavage, and commonly magnetic if magnetite-rich; specific gravity is roughly 3.0–4.5.

Identified September 12, 2026More metamorphic →

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Physical properties

Formation & geological history

Chemical sediment deposited in Precambrian oceans, chiefly about 3.8–1.8 billion years ago, when dissolved iron precipitated as oxygen levels rose. Later burial and metamorphism produced the dense, folded or fractured banding.

Uses & applications

Major global source of iron ore after beneficiation; attractive polished pieces are used as lapidary stone, décor, and educational specimens.

Geological facts

Alternating silica and iron-rich bands record major changes in Earth’s early atmosphere and ocean chemistry. Rusty surfaces reflect oxidation of iron minerals.

Field identification & locations

Identify by thin, repetitive dark iron-rich and pale chert bands, high density, and possible attraction to a magnet; common in the Lake Superior region, Western Australia, Brazil, India, and South Africa.