sedimentary
Limitation Concretion
Ironstone Concretion (Goethite/Limonite)
Hardness: 4.0-5.5 (Mohs scale); Color: Ochre, rusty brown to tan; Luster: Dull/Earthy; Structure: Amorphous or microcrystalline; Cleavage: None (conchoidal fracture); Heavy for its size.
- Hardness
- 4
- Color
- Ochre, rusty brown to tan
- Luster
- Dull/Earthy
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Physical properties
Hardness: 4.0-5.5 (Mohs scale); Color: Ochre, rusty brown to tan; Luster: Dull/Earthy; Structure: Amorphous or microcrystalline; Cleavage: None (conchoidal fracture); Heavy for its size.
Formation & geological history
Formed by the precipitation of mineral cement, likely iron oxides, within the spaces between sediment grains. These often form in sedimentary rock layers like sandstone or shale during early diagenesis.
Uses & applications
Primarily collected as geological curiosities due to their unusual shapes; sometimes used as minor iron ore or crushed for pigments.
Geological facts
Concretions are often mistaken for fossils, bones, or meteorites due to their rounded, biological-looking shapes. The process of formation can take thousands to millions of years.
Field identification & locations
Identified by its heavy weight relative to its size and its distinct earthy, rusty color. Look for them in eroding cliffs, riverbeds, and sedimentary basins. If broken open, they sometimes reveal a target-like banding or a different core.
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