sedimentary
Ironstone Concretion
Siderite or Hematite Concretion (FeCO3 or Fe2O3)
Hardness: 5-6 (Mohs); Color: Dark brown to rusty reddish-brown exterior, often with a lighter core; Luster: Dull, earthy; Crystal structure: Usually microcrystalline or amorphous; Cleavage: None; Specific Gravity: 3.5 - 4.5.
- Hardness
- 5-6 (Mohs)
- Luster
- Dull, earthy
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark brown to rusty reddish-brown exterior, often with a lighter core; Luster: Dull, earthy; Crystal structure: Usually microcrystalline or amorphous; Cleavage: None; Specific Gravity: 3.5 - 4.5.
Formation & geological history
Formed by the precipitation of iron oxide or carbonate minerals around a nucleus (like a leaf, shell, or pebble) within sediment layers. This process typically occurs shortly after the sediment was deposited but before it became rock, often in swampy or shallow marine environments during the Carboniferous to Cretaceous periods.
Uses & applications
Historically used as a low-grade iron ore. Today, they are primarily collected by enthusiasts as curiosities, used in decorative landscaping, or occasionally as paperweights.
Geological facts
Many concretions are mistaken for meteorites or fossils due to their unusual shapes. When split open, they sometimes reveal perfectly preserved fossils inside, such as those found in the famous Mazon Creek formation.
Field identification & locations
Identify by its heavy weight relative to size (due to iron content), its rounded or teardrop shape, and often concentric layering visible on the surface. Commonly found in eroded shale beds, riverbanks, and coastal cliffs.
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