sedimentary
Ironstone Nodule
Ferruginous Siltstone/Sandstone
Hardness: 5.0-6.5 (variable); Color: Dark brown, reddish-brown, or rusty black; Luster: Dull, earthy; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 3.0-3.8 (heavier than average rock).
- Hardness
- 5
- Color
- Dark brown, reddish-brown, or rusty black
- Luster
- Dull, earthy
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Physical properties
Hardness: 5.0-6.5 (variable); Color: Dark brown, reddish-brown, or rusty black; Luster: Dull, earthy; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 3.0-3.8 (heavier than average rock).
Formation & geological history
Formed through the precipitation of iron oxides (like goethite or hematite) within sedimentary layers, often during the diagenesis of silt or sand. They frequently form around a nucleus in marshy or marine environments rich in iron-bearing groundwater.
Uses & applications
Primarily used as a minor iron ore historically; currently valued as decorative garden stones or for geological study and fossil hunting, as they sometimes contain internal fossils.
Geological facts
Ironstones are responsible for the 'red' colors seen in many famous landscapes like the American Southwest. They are remarkably resistant to weathering, which is why they often sit on the surface of the ground long after the surrounding softer rock has eroded away.
Field identification & locations
Identify by its surprising weight compared to its size and its dark, rusty exterior. It will often produce a reddish or brown streak when rubbed on unglazed porcelain. Commonly found in eroded creek beds or fields where shale and sandstone are present.
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