sedimentary
Ironstone concretion
Siderite or Limonite-rich Concretion (FeO(OH)·nH2O)
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
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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.
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