sedimentary
Ironstone Concretions (Ironstone)
Ferruginous Sandstone or Siltstone (Fe2O3)
Hardness: 5-6 (Mohs scale), Color: Dark brown, reddish-brown, or blackish, Luster: Dull to sub-metallic, Structure: Amorphous or microcrystalline, Cleavage: None, SG: 3.0-4.5
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Physical properties
Hardness: 5-6 (Mohs scale), Color: Dark brown, reddish-brown, or blackish, Luster: Dull to sub-metallic, Structure: Amorphous or microcrystalline, Cleavage: None, SG: 3.0-4.5
Formation & geological history
Formed via the precipitation of iron-oxide minerals within sedimentary basins, typically when groundwater rich in mineral content reacts with sediment during diagenesis.
Uses & applications
Primary source of iron ore for smelting; also used as decorative gravel, in traditional pigments (ochre), and for historical construction in specific regions.
Geological facts
Often called 'Moqui Marbles' or 'Blueberry Rocks' when found in spherical shapes; similar iron-rich concretions have been discovered on Mars by the Opportunity rover.
Field identification & locations
Identify in the field by their extreme density compared to surrounding sandstone, rust-colored streak test, and often irregular, pitted, or smooth pebbled surfaces. Common in arid desert regions.
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