sedimentary
Ferruginous Sandstone (Gritstone)
Clastic sedimentary rock with iron-rich cement (FeO(OH)·nH2O)
Hardness: 6-7 (Mohs); Color: Dark brown to reddish-orange; Luster: Dull/Earthy; Crystal Structure: Clastic/Granular; Cleavage: None; Specific Gravity: 2.5-3.0.
- Hardness
- 6-7 (Mohs)
- Color
- Dark brown to reddish-orange
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark brown to reddish-orange; Luster: Dull/Earthy; Crystal Structure: Clastic/Granular; Cleavage: None; Specific Gravity: 2.5-3.0.
Formation & geological history
Formed through the accumulation of sand-sized grains (mostly quartz) that were later cemented together by iron oxides like hematite or goethite precipitated from groundwater. Typically found in terrestrial or shallow marine environments.
Uses & applications
Commonly used in construction as an aggregate, road metal, or building stone. Historically used for millstones. It is also of interest to collectors of 'ironstones'.
Geological facts
Also known as ironstone in some contexts, these rocks can contain high enough iron content to be considered a low-grade iron ore. Their distinct reddish-brown color comes from the oxidation (rusting) of iron minerals.
Field identification & locations
Identify by its gritty sandpaper feel, heavy weight for its size (due to iron content), and reddish-brown streak. Common in areas with sedimentary basin deposits.
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