metamorphic
Quartzite Cobble
Quartzite (mostly SiO2)
Hardness: 7 on Mohs scale; Color: Gray, reddish-brown, and tan mottled; Luster: Dull to vitreous; Crystal structure: Granoblastic (recrystallized quartz grains); Cleavage: None (conchoidal fracture).
- Hardness
- 7 on Mohs scale
- Color
- Gray, reddish-brown, and tan mottled
- Luster
- Dull to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, reddish-brown, and tan mottled; Luster: Dull to vitreous; Crystal structure: Granoblastic (recrystallized quartz grains); Cleavage: None (conchoidal fracture).
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone under intense heat and pressure. This specific rounded shape indicates years of fluvial (river) or glacial transport, wearing down the edges into a smooth cobble.
Uses & applications
Commonly used as crushed stone in construction, road ballast, and occasionally as decorative landscaping stone.
Geological facts
Quartzite is so tough that it often survives environmental weathering better than the rocks originally surrounding it. It is often harder than the steel of a pocket knife.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. Often found in riverbeds, glacial tills, and mountainous metamorphic belts.
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