sedimentary
Red Sandstone (Siltstone/Mudstone variety)
Ferruginous Sandstone (composed primarily of Quartz with Hematite cementation)
Hardness: 6-7 (Mohs scale for grains, lower for matrix); Color: Reddish-brown, maroon, or rust-red; Luster: Dull or earthy; Crystal structure: Granular/Clastic; Cleavage: None (conchoidal or irregular fracture); Specific gravity: 2.2 - 2.8
- Hardness
- 6-7 (Mohs scale for grains, lower for matrix)
- Color
- Reddish-brown, maroon, or rust-red
- Luster
- Dull or earthy
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Physical properties
Hardness: 6-7 (Mohs scale for grains, lower for matrix); Color: Reddish-brown, maroon, or rust-red; Luster: Dull or earthy; Crystal structure: Granular/Clastic; Cleavage: None (conchoidal or irregular fracture); Specific gravity: 2.2 - 2.8
Formation & geological history
Formed from the accumulation and lithification of sand, silt, and clay particles in terrestrial environments like riverbeds or deserts. The red color suggests deposition in oxygen-rich environments where iron minerals oxidized into hematite. Geological age varies but often associated with Devonian or Triassic 'Old Red Sandstone' formations.
Uses & applications
Commonly used as a decorative building stone, crushed stone for road construction, landscape gravel, and occasionally for minor lapidary art like garden stones.
Geological facts
The distinct red color is due to 'iron staining,' essentially rust. Famous formations include the Grand Canyon's layers and the red rocks of Sedona and Arches National Park. It has been used for centuries as a primary building material in historical cathedrals and castles.
Field identification & locations
Identify in the field by its gritty texture (feels like sandpaper), earthy smell when wet, and lack of visible crystals. It is ubiquitous in sedimentary basins and along coastlines near uplifted red-bed formations. Collectors look for smooth 'river-tumbled' specimens or those with interesting ripples.
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