sedimentary
Ferruginous Sandstone (Ironstone)
Arenite with Iron-Oxide Cement (FeO(OH)·nH2O / Fe2O3)
Hardness: 6-7 on Mohs scale (due to quartz content); Color: Reddish-brown, ochre, or dark rusty chocolate; Luster: Dull to earthy; Crystal structure: Granular/Clastic; Specific Gravity: 2.0-3.0 depending on iron concentration.
- Hardness
- 6-7 on Mohs scale (due to quartz content)
- Color
- Reddish-brown, ochre, or dark rusty chocolate
- Luster
- Dull to earthy
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Physical properties
Hardness: 6-7 on Mohs scale (due to quartz content); Color: Reddish-brown, ochre, or dark rusty chocolate; Luster: Dull to earthy; Crystal structure: Granular/Clastic; Specific Gravity: 2.0-3.0 depending on iron concentration.
Formation & geological history
Formed through the lithification of sand grains where iron oxides (like hematite or goethite) act as the primary cementing agent. These often form in terrestrial, fluvial, or shallow marine environments during the Phanerozoic eon.
Uses & applications
Commonly used as a building stone (brownstone), road aggregate, or decorative landscaping rock. In some cases, it serves as a low-grade iron ore.
Geological facts
Many of the world's most famous desert landscapes and 'red rocks' owe their color to the same iron-oxide staining seen in this specimen. It can preserve fossils, although the high acidity during formation sometimes destroys organic matter.
Field identification & locations
Identify by its characteristic 'rust' color that does not wash off and its gritty sandpaper feel. Frequently found in regions with exposed sedimentary beds such as the Southwestern US, UK, and Australia. Collectors look for unique weathering patterns or 'desert varnish' on the surface.
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