metamorphic
River Cobble (Quartzite)
Meta-orthoquartzite
Hardness: 7 (Mohs scale); Color: Gray to brownish with tan highlights; Luster: Dull to waxy; Structure: Crystalline/Granular; Cleavage: None (conchoidal fracture).
- Hardness
- 7 (Mohs scale)
- Color
- Gray to brownish with tan highlights
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray to brownish with tan highlights; Luster: Dull to waxy; Structure: Crystalline/Granular; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed from the metamorphism of quartz sandstone under high heat and pressure, then rounded by fluvial erosion in a riverbed environment over thousands of years.
Uses & applications
Used primarily in landscaping, decorative ground cover, construction aggregate, and as a natural specimen for rock painting or education.
Geological facts
Quartzite is so durable that it can withstand the intense weathering of river transport better than almost any other rock. Many river cobbles are much older than the riverbed they are found in.
Field identification & locations
Identify by its extreme hardness (scratches glass) and its rounded, water-worn shape. Commonly found in riverbeds, glacial tills, and alluvial fans.
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