sedimentary
Ironstone Concretion
Ironstone (primarily Siderite or Limonite)
Hardness: 4 to 5.5 on Mohs scale; Color: Dark brown, reddish-brown, or rusty; Luster: Dull, earthy, or metallic; Structure: Often rounded or nodular; Specific Gravity: 3.0 to 3.9.
- Hardness
- 4 to 5
- Color
- Dark brown, reddish-brown, or rusty
- Luster
- Dull, earthy, or metallic
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Physical properties
Hardness: 4 to 5.5 on Mohs scale; Color: Dark brown, reddish-brown, or rusty; Luster: Dull, earthy, or metallic; Structure: Often rounded or nodular; Specific Gravity: 3.0 to 3.9.
Formation & geological history
Formed by the precipitation of mineral cement, typically iron oxides, within the spaces between sediment grains. These often grow around a nucleus of organic material or shell fragments in sedimentary basins.
Uses & applications
Historically used as a low-grade iron ore. Mostly a novelty item for collectors or used in decorative landscaping and garden features.
Geological facts
Many ironstones exhibit a 'pseudo-fossil' appearance, mimicking the shape of nuts, bones, or eggs, which often leads people to mistake them for actual fossils.
Field identification & locations
Can be identified by their high density compared to surrounding rocks and their tendency to have a 'shell' or layered appearance when broken. Very common in clay-rich sedimentary environments.
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