metamorphic
Quartzite Cobble
Metamorphosed Quartz Sandstone (SiO2)
Hardness: 7 (Mohs scale); Color: Whitehead, grey, tan with minor iron staining; Luster: Vitreous to dull; Crystal structure: Non-foliated granoblastic; Cleavage: None (conchoidal fracture)
- Hardness
- 7 (Mohs scale)
- Color
- Whitehead, grey, tan with minor iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Whitehead, grey, tan with minor iron staining; Luster: Vitreous to dull; Crystal structure: Non-foliated granoblastic; Cleavage: None (conchoidal fracture)
Formation & geological history
Formed when quartz-rich sandstone is subjected to heat, pressure, and chemical activity related to tectonic compression. This specific rounded shape indicates water erosion in a fluvial (river) or glacial environment.
Uses & applications
Used as a construction aggregate, road ballast, and in landscaping. Pure quartzite is also used in the glass and ceramic industries.
Geological facts
Quartzite is so durable that it can survive multiple cycles of erosion and redeposition. It is often harder than the steel tools used to mine it.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. In the field, it is commonly found as rounded river stones or in mountainous outcrops.
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