metamorphic
Quartzite (River Cobble)
Quartzite (Metamorphosed Quartz Sandstone, SiO2)
Hardness: 7 (Mohs scale); Color: Pale grey, white, often with tan or reddish staining; Luster: Dull to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.7
- Hardness
- 7 (Mohs scale)
- Color
- Pale grey, white, often with tan or reddish staining
- Luster
- Dull to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Pale grey, white, often with tan or reddish staining; Luster: Dull to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.7
Formation & geological history
Formed from the recrystallization of quartz-rich sandstone through heat and pressure (regional metamorphism). This specific specimen has been further shaped into a rounded cobble by water erosion in a high-energy fluvial (river) environment.
Uses & applications
Commonly used as crushed stone for road construction, railway ballast, and landscaping. High-purity quartzite is used to produce ferrosilicon, industrial silica sand, and silicon metal.
Geological facts
Quartzite is so durable that it often survives multiple cycles of erosion and redeposition. It is one of the hardest common rocks and was historically used by prehistoric humans to create stone tools because of its conchoidal fracturing properties.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch glass and steel) and its granular, sugary texture on a fresh break. Commonly found in riverbeds, glacial tills, and mountainous metamorphic belts worldwide.
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