sedimentary
River Cobble (Quartzite or Weathered Sandstone)
Arenite / Metasandstone (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Tan, beige, and dark grey mottling; Luster: Dull to sub-vitreous when wet; Structure: Granular or microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6
- Color
- Tan, beige, and dark grey mottling
- Luster
- Dull to sub-vitreous when wet
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Tan, beige, and dark grey mottling; Luster: Dull to sub-vitreous when wet; Structure: Granular or microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the lithification of sand deposits, likely subjected to river transport which rounded its edges into a 'cobble' shape. May have experienced low-grade metamorphism (quartzite) depending on its depth of burial and heat exposure.
Uses & applications
Used primarily as decorative landscaping stone, aggregate for construction, or as river rock for drainage and erosion control. Occasionally collected for lapidary tumbling.
Geological facts
The rounded shape indicates a long history of hydraulic transport, where the rock was tumbled against others in a stream bed, smoothing its sharp edges. The pitting on the surface suggests the erosion of softer mineral grains or dissolution over time.
Field identification & locations
Identify in the field by its rounded, water-worn shape and high hardness (cannot be scratched by a steel knife). Commonly found in current or ancient riverbeds, alluvial fans, and coastal regions.
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