sedimentary
Ironstone Concretion (Pot Stone)
Siderite or Limonite Concretion (FeCO3 or FeO(OH)·nH2O)
Hardness: 4.0-5.5 on Mohs scale; Color: Rusty brown, ochre, or reddish-tan; Luster: Dull/Earthy; Crystal Structure: Cryptocrystalline; Cleavage: None/Irregular; Specific Gravity: 3.0-3.9
- Hardness
- 4
- Color
- Rusty brown, ochre, or reddish-tan
- Luster
- Dull/Earthy
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Physical properties
Hardness: 4.0-5.5 on Mohs scale; Color: Rusty brown, ochre, or reddish-tan; Luster: Dull/Earthy; Crystal Structure: Cryptocrystalline; Cleavage: None/Irregular; Specific Gravity: 3.0-3.9
Formation & geological history
Formed by the precipitation of mineral cement (iron oxides or carbonates) within the spaces between sediment grains, often around a nucleus like a shell or leaf. Typical of the Cretaceous or Carboniferous periods in swampy or marine environments.
Uses & applications
Primarily for geological study, education, or as garden ornaments and curiosities. Historically, some iron-rich concretions were used as low-grade iron ore.
Geological facts
These are often mistaken for man-made bowls, fossilized dinosaur eggs, or ancient artifacts due to their symmetrical 'pot' shape. They are actually entirely natural geological formations.
Field identification & locations
Identify in the field by their rounded, bowl-like or spherical shapes and heavy weight relative to size. Commonly found in eroded sandstone or clay beds in the UK, USA (Ohio, Kansas), and Australia.
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