sedimentary
Ironstone Concretion
Ferruginous Sandstone / Hematite-rich Concretion
Hardness: 5-6 (Mohs scale), Color: Dark brown to reddish-brown or rusty orange, Luster: Dull to sub-metallic, Crystal Structure: Amorphous or microcrystalline (aggregates), Specific Gravity: 3.5-5.0
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Physical properties
Hardness: 5-6 (Mohs scale), Color: Dark brown to reddish-brown or rusty orange, Luster: Dull to sub-metallic, Crystal Structure: Amorphous or microcrystalline (aggregates), Specific Gravity: 3.5-5.0
Formation & geological history
Formed by the precipitation of iron oxide minerals (like hematite or goethite) within the pore spaces of sedimentary rocks, typically sandstone or shale. This chemical process often occurs during diagenesis as groundwater rich in iron moves through the sediment.
Uses & applications
Historically used as a low-grade iron ore; commonly collected by rockhounds for its unusual shapes; occasionally used as a decorative landscaping stone or crushed for road base.
Geological facts
These stones are often mistaken for meteorites because of their dark 'fusion crust' look and high density, but they are purely terrestrial. Some concretions contain fossils at their core, which acted as the chemical trigger for their formation.
Field identification & locations
Identify by its heavy weight compared to other rocks and its characteristic rusty rub-off or streak. Found globally in sedimentary basins, especially in areas with iron-rich groundwater. Look for their rounded or tabular shapes protruding from eroding cliffs or creek beds.
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