sedimentary

Limonite Concretion (Ironstone)

Limonite (FeO(OH)·nH2O) / Goethite

AI-generated identification · may be incorrect

Hardness: 4.0-5.5 (Mohs scale), Color: Rusty brown to dark brown, Luster: Dull to sub-metallic, Crystal Structure: Amorphous/Microcrystalline, Specific gravity: 2.7-4.3

Hardness: 4

Identified March 29, 2026More sedimentary →

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Physical properties

Hardness: 4.0-5.5 (Mohs scale), Color: Rusty brown to dark brown, Luster: Dull to sub-metallic, Crystal Structure: Amorphous/Microcrystalline, Specific gravity: 2.7-4.3

Formation & geological history

Formed through the chemical precipitation of iron-rich minerals in sedimentary rock layers or soils. Often found as nodules or 'ironstone' pebbles shaped by water erosion in stream beds or coastal areas.

Uses & applications

Historically used as an iron ore; also used as a natural pigment (brown ochre) and as decorative landscape gravel or ornamental stones for collectors.

Geological facts

Limonite is not a true mineral but a mixture of hydrated iron oxide minerals. It often forms 'Moqui Marbles' or similar concretions which are sometimes mistaken for meteorites due to their weight and color.

Field identification & locations

Identify by its high density relative to other common stones, a rusty brown streak when scratched against unglazed porcelain, and lack of magnetism (unlike magnetite). Common in areas with iron-rich sandstone.