metamorphic
Quartzite (River Cobble)
Quartzite (Metamorphosed Quartz Sandstone, SiO2)
Hardness: 7 on Mohs scale (scratches glass). Color: Milky white to light grey with occasional iron staining. Luster: Vitreous to waxy when polished; dull when weathered. Structure: Non-foliated, granular. Cleavage: None (conchoidal fracture).
- Hardness
- 7 on Mohs scale (scratches glass)
- Color
- Milky white to light grey with occasional iron staining
- Luster
- Vitreous to waxy when polished
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Physical properties
Hardness: 7 on Mohs scale (scratches glass). Color: Milky white to light grey with occasional iron staining. Luster: Vitreous to waxy when polished; dull when weathered. Structure: Non-foliated, granular. Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the regional metamorphism of quartz-rich sandstone. Under high heat and pressure, the individual sand grains recrystallize and fuse together. The rounded shape indicates significant water erosion, likely millions of years of transport in a riverbed or coastal environment.
Uses & applications
Used in the construction industry as crushed stone for road gravel and railway ballast. Exceptionally pure quartzite is used to produce silica sand for glassmaking and silicon for electronics. Rounded river cobbles are frequently used in landscaping and masonry.
Geological facts
Quartzite is so tough that it often outlasts the mountain ranges from which it originated. Unlike sandstone, which breaks around sand grains, quartzite is so well-fused that it breaks through the grains. It is often confused with marble, but quartzite will not react to acid and is much harder.
Field identification & locations
Identify in the field by its inability to be scratched by a steel knife and its sugary, granular texture on fresh breaks. It is ubiquitous in riverbeds, glacial tills, and mountainous terrains worldwide. Collectors find value in specimens with unique inclusions or high translucent clarity.
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