sedimentary
Ironstone concretion / Limonitic Sandstone
Goethite/Hematite Cemented Sandstone (Hydrated Iron Oxide)
Hardness: 5-5.5 (mohs); Color: Rusty brown, reddish-orange, tan; Luster: Dull/Earthy; Crystal Structure: Amorphous or Microcrystalline; Cleavage: None; Specific Gravity: 3.3-4.3
- Hardness
- 5-5
- Color
- Rusty brown, reddish-orange, tan
- Luster
- Dull/Earthy
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Physical properties
Hardness: 5-5.5 (mohs); Color: Rusty brown, reddish-orange, tan; Luster: Dull/Earthy; Crystal Structure: Amorphous or Microcrystalline; Cleavage: None; Specific Gravity: 3.3-4.3
Formation & geological history
Formed through the precipitation of iron-rich minerals (limonite, goethite) within the pore spaces of sedimentary sand. This usually occurs due to groundwater movement or oxidation in bog/wetland environments.
Uses & applications
Historically used as a low-grade iron ore (bog iron). Currently used as landscaping stone, natural pigments (ochre), or as a curiosity/educational specimen for amateur collectors.
Geological facts
These are often mistaken for meteorites or fossils (like coprolites) due to their unusual organic shapes and heavy weight. They can form around a nucleus of organic matter like a leaf or shell.
Field identification & locations
Identify by its 'rusty' appearance and relatively high weight compared to normal sandstone. Commonly found in areas with high iron content in groundwater or exposed sedimentary beds. It will leave a yellow-brown streak on a porcelain plate.
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