sedimentary

Tumbled banded iron formation (jasper-rich ironstone)

Banded iron formation, commonly hematite/magnetite with microcrystalline quartz (SiO₂)

AI-generated identification · may be incorrect

Hardness about 6–7 (quartz) and up to 6.5 (hematite); reddish-brown, dark gray, and rusty bands with a dull to waxy or slightly metallic luster. The specimen is rounded and polished by natural or mechanical tumbling; original quartz layers are microcrystalline rather than visibly crystalline.

Identified August 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Banded iron formations formed mainly in Precambrian oceans, roughly 3.8–1.8 billion years ago, when dissolved iron precipitated with silica during rising atmospheric oxygen. Weathering and later tumbling exposed the layered iron-rich texture.

Uses & applications

Used as iron ore where sufficiently rich; jasper varieties are popular for lapidary work, cabochons, carvings, and inexpensive decorative specimens.

Geological facts

The reddish coloration generally comes from hematite, while darker layers may contain magnetite or iron-rich silicates. A streak test can distinguish hematite (red-brown streak) from magnetite (black streak and strong magnetism).

Field identification & locations

Identify by dense weight, fine parallel banding, red-brown coloration, and possible weak-to-strong magnetism; common sources include Minnesota, Michigan, Western Australia, Brazil, and South Africa. Value: typically $3–$15 for a tumbled piece of this size, or $20–$75 for especially attractive polished material.