sedimentary
Ironstone Concretion (Limonite/Hematite Nodules)
Ironstone (principally Limonite FeO(OH)·nH2O or Hematite Fe2O3 mixed with silica/clay)
Hardness: 4.0-5.5 (Mohs scale); Color: Rusty brown, reddish-orange, or dark ochre; Luster: Dull, earthy; Crystal structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 2.7–4.3 depending on iron concentration.
- Hardness
- 4
- Color
- Rusty brown, reddish-orange, or dark ochre
- Luster
- Dull, earthy
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Physical properties
Hardness: 4.0-5.5 (Mohs scale); Color: Rusty brown, reddish-orange, or dark ochre; Luster: Dull, earthy; Crystal structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 2.7–4.3 depending on iron concentration.
Formation & geological history
Formed by the precipitation of iron-rich minerals in the spaces between sediment grains (usually sandstone or shale). These often form around a nucleus of organic material or a different mineral within sedimentary basins. They can range from millions to several hundred million years old.
Uses & applications
Historically used as a low-grade iron ore. Mostly used today for decorative landscaping, small carvings, or as educational geological specimens (popularly known as 'Moqui Marbles' or 'Shaman Stones' in specific regions).
Geological facts
They are often mistaken for meteorites due to their heavy weight and dark, weathered crust. Some concretions contain hollow centers or 'rattlestones' where the interior material has dried and detached from the outer shell.
Field identification & locations
Identify by the heavy weight for its size, rusty streak when rubbed on unglazed porcelain, and distinctive rounded or potato-like shape. Commonly found in eroded sandstone landscapes, stream beds, and desert environments worldwide.
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