sedimentary

Ironstone concretion

Siderite or Limonite-rich Concretion (FeO(OH)·nH2O)

AI-generated identification · may be incorrect

Hardness: 4.0-5.5 (Mohs), Color: Dark brown to rusty orange-brown, Luster: Dull or earthy, Structure: Amorphous or microcrystalline, Cleavage: None, Specific Gravity: 2.7-4.3

Hardness: 4

Identified July 21, 2026More sedimentary →

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Physical properties

Hardness: 4.0-5.5 (Mohs), Color: Dark brown to rusty orange-brown, Luster: Dull or earthy, Structure: Amorphous or microcrystalline, Cleavage: None, Specific Gravity: 2.7-4.3

Formation & geological history

Formed by the precipitation of iron-rich mineral cement within the spaces between sediment grains (usually sand or silt). Often found in sedimentary rock layers like shale or sandstone, dating from various geological eras.

Uses & applications

Primarily used as decorative lawn or garden stones, or for geological collection; historically sometimes processed as a low-grade iron ore.

Geological facts

These are often mistaken for meteorites due to their heavy weight and dark, weathered appearance. They can contain fossils or smaller nodules inside if broken open.

Field identification & locations

Identify by its high density compared to surrounding rocks and its rusty streak when rubbed on unglazed porcelain. Commonly found in riverbeds or eroding cliff sites in Minnesota and the Midwest.