sedimentary
Ferruginous Sandstone (Ironstone Nodules)
Ferruginous Sandstone with Limonite/Goethite coating (Fe2O3·H2O)
Hardness: 6-7 (Mohs scale) for the silica base; Color: Reddish-brown, tan, and gray; Luster: Dull/Earthy; Structure: Clastic, granular texture; Cleavage: None; Specific Gravity: 2.5-3.0
- Hardness
- 6-7 (Mohs scale) for the silica base
- Color
- Reddish-brown, tan, and gray
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs scale) for the silica base; Color: Reddish-brown, tan, and gray; Luster: Dull/Earthy; Structure: Clastic, granular texture; Cleavage: None; Specific Gravity: 2.5-3.0
Formation & geological history
Formed through the accumulation and dewatering of sand-sized minerals (mostly quartz) in riverbeds or marine environments, then cemented together by iron oxide minerals precipitated from groundwater. This specific specimen shows weathering in an oxidizing environment.
Uses & applications
Commonly used as a base material in construction, road fill, and occasionally as a low-grade iron ore. In the lapidary world, it is sometimes used for yard decoration or rustic stone carving.
Geological facts
The reddish-brown color is essentially 'geological rust'. These rocks are often formed into 'Moki Marbles' or 'Concretions' that can have unusual spherical or layered shapes, leading people to mistake them for fossils or meteorites.
Field identification & locations
Identify by its gritty texture and characteristic orange-brown 'rust' staining. It will not scratch with a copper penny but will usually be scratched by a steel file. Commonly found in deserts or river banks in the American Southwest and worldwide.
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