sedimentary
Ironstone Concretion (Limonite/Goethite)
Ironstone (composed primarily of Goethite FeO(OH) and Limonite)
Hardness: 4.0-5.5; Color: Dark brown, reddish-brown, to earthy yellow; Luster: Dull to sub-metallic; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 2.7-4.3
- Hardness
- 4
- Color
- Dark brown, reddish-brown, to earthy yellow
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 4.0-5.5; Color: Dark brown, reddish-brown, to earthy yellow; Luster: Dull to sub-metallic; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 2.7-4.3
Formation & geological history
Formed through the precipitation of iron-rich minerals within sedimentary rock layers, often around a nucleus like a leaf, shell, or fossil. These typically form in swampy, boggy, or shallow marine environments over millions of years.
Uses & applications
Historically used as a low-grade iron ore. Today, primarily of interest to geological collectors and used in decorative landscaping.
Geological facts
Concretions are often mistaken for meteorites or fossils due to their unusual shapes and heavy weight. They are harder than the surrounding sandstone or shale and remain after the host rock has weathered away.
Field identification & locations
Identify by its high density (feels heavy for its size), rusty streak test, and lack of internal crystalline structure. Common in sedimentary basins and along desert floors where erosion has exposed them.
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