sedimentary
Ironstone Concretion
Siderite or Hematite Concretion
Hardness: 5-6 (Mohs); Color: Tan/yellow outer shell with a dark reddish-brown iron-rich core; Luster: Dull/Earthy; Structure: Nodular/Amorphous; Cleavage: None; Specific Gravity: 3.0-3.9
- Hardness
- 5-6 (Mohs)
- Luster
- Dull/Earthy
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Physical properties
Hardness: 5-6 (Mohs); Color: Tan/yellow outer shell with a dark reddish-brown iron-rich core; Luster: Dull/Earthy; Structure: Nodular/Amorphous; Cleavage: None; Specific Gravity: 3.0-3.9
Formation & geological history
Formed through the precipitation of mineral cement (typically iron oxides or carbonates) in the spaces between sediment grains, often around a nucleus such as a leaf, bone, or shell. These typically form in sedimentary basins over millions of years.
Uses & applications
Primarily used as geological curiosities for collectors; historically some high-iron concretions were used as iron ore. Some are used in landscaping or decorative masonry.
Geological facts
Many people mistake the interior patterns of broken concretions for fossils or organic remains like hearts or organs due to their distinct color zoning. This example shows an iron-rich 'nucleus' interior inside a sandy outer rind.
Field identification & locations
Identify by looking for rounded, pebble-like stones that appear heavier than they look and show concentric bands of different colors when broken. Common in riverbeds and eroded sandstone cliffs.
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