sedimentary
River Cobble (Quartzite / Sandstone Gravel)
SiO2 (Quartz-rich sedimentary/metamorphic rock)
Hardness: 6-7 Mohs. Color: Tan, brown, reddish-brown, grey. Luster: Dull to vitreous when wet. Texture: Smooth, well-rounded, micro-pitted surface from water erosion. Specific gravity: 2.6-2.7.
- Hardness
- 6-7 Mohs
- Color
- Tan, brown, reddish-brown, grey
- Luster
- Dull to vitreous when wet
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Physical properties
Hardness: 6-7 Mohs. Color: Tan, brown, reddish-brown, grey. Luster: Dull to vitreous when wet. Texture: Smooth, well-rounded, micro-pitted surface from water erosion. Specific gravity: 2.6-2.7.
Formation & geological history
Formed through the mechanical weathering, transportation, and continuous abrasion of quartz-rich rocks in riverbeds, streams, or glacial meltwater channels over thousands to millions of years.
Uses & applications
Used extensively in landscaping, river rock garden beds, aquarium decor, construction aggregate, concrete mixing, and outdoor paving.
Geological facts
Smooth river stones acquire their rounded shape from collisions with other rocks and sand in moving water, a geological process called attrition.
Field identification & locations
Identified by its egg-like or flattened oval shape, lack of sharp edges, and dense composition. Extremely common along river banks, lake shores, and glacial deposits globally.
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