metamorphic
Quartzite Cobble
Meta-quartzite (90%+ SiO2)
Hardness: 7 (Mohs scale), Color: Brownish-tan to grey (due to iron oxide staining), Luster: Dull to vitreous when wet, Crystal structure: Granoblastic, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: Brownish-tan to grey (due to iron oxide staining), Luster: Dull to vitreous when wet, Crystal structure: Granoblastic, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the metamorphism of quartz sandstone under high heat and pressure, causing quartz grains to recrystallize and fuse. This specimen is a rounded cobble, shaped by long-term fluvial or glacial transport and wave action.
Uses & applications
Commonly used as crushed stone in road construction, railway ballast, and as high-quality decorative river rock for landscaping.
Geological facts
Quartzite is so durable that it often outlasts the mountains it was formed in, becoming the primary component of many river bed and beach cobbles. It is harder than steel and will easily scratch glass.
Field identification & locations
Identify by its extreme hardness and grainy texture that looks like sandpaper but is smooth to the touch. Look for the 'sugary' sparkle of fused quartz grains on a fresh break. Found worldwide in ancient shield areas and river basins.
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