sedimentary

Ironstone Concretion

Ferruginous Concretion

Hardness: 5-5.5 (Mohs scale); Color: Brown, reddish-brown, dark grey, or ocher; Luster: Dull to sub-metallic; Crystal Structure: Amorphous/massive; Cleavage: None; Specific Gravity: 3.0-4.5 (heavy due to iron content).

Hardness: 5-5
Color: Brown, reddish-brown, dark grey, or ocher
Luster: Dull to sub-metallic

Identified July 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of iron-oxide minerals (like goethite or limonite) between sediment grains, often around a central nucleus such as a leaf, bone, or shell. They typically form in sedimentary basins over millions of years.

Uses & applications

Primarily of interest to collectors and geologists; historically used as a low-grade iron ore in small-scale smelting.

Geological facts

These are often mistaken for meteorites because of their dark, heavy, and weathered appearance. They can contain well-preserved fossils inside if cracked open carefully.

Field identification & locations

Identify by its 'rind-like' layers, heavy weight, and reddish streak. Commonly found in sandstone or shale beds, often weathering out as rounded nodules. Found worldwide in locations like the Great Plains (US) and the UK.