metamorphic
Quartzite (River Cobble)
Metamorphosed Quartz Sandstone (SiO2)
Hardness: 7 on Mohs scale; Color: White to light tan/grey; Luster: Vitreous to dull; Structure: Non-foliated granoblastic; Fracture: Conchoidal to irregular; Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- White to light tan/grey
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: White to light tan/grey; Luster: Vitreous to dull; Structure: Non-foliated granoblastic; Fracture: Conchoidal to irregular; Specific Gravity: 2.65.
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone under intense heat and pressure, causing the original quartz grains to recrystallize. This specific specimen has been rounded into a cobble by mechanical weathering in a river or coastal environment.
Uses & applications
Commonly used as crushed stone in road construction, railroad ballast, and as a raw material in glass manufacturing. In smooth cobble form, it is popular for decorative landscaping and stone masonry.
Geological facts
Quartzite is so tough that it often outlasts the mountain ranges it formed in, remaining as resistant outcrops or debris long after other rocks have eroded away. It is often mistake for marble, but can be distinguished because quartzite will scratch glass while marble will not.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by an iron nail) and its sugary, crystalline texture when broken. It is a common constituent of riverbeds and glacial till globally.
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