metamorphic
Quartzite River Cobble
Quartzite (SiO2)
Hardness: 7 (Mohs scale); Color: Tan, brownish-orange due to iron staining; Luster: Vitreous to dull; Crystal structure: Non-foliated, granular; Cleavage: None, breaks with conchoidal or irregular fracture; Specific gravity: 2.6–2.7.
- Hardness
- 7 (Mohs scale)
- Color
- Tan, brownish-orange due to iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, brownish-orange due to iron staining; Luster: Vitreous to dull; Crystal structure: Non-foliated, granular; Cleavage: None, breaks with conchoidal or irregular fracture; Specific gravity: 2.6–2.7.
Formation & geological history
Formed from the regional or contact metamorphism of quartz sandstone. Intense heat and pressure cause quartz grains to recrystallize and fuse together. The rounded shape is the result of mechanical weathering and transport in high-energy river or beach environments.
Uses & applications
Used primarily in construction for road ballast, railroad beds, and decorative landscaping. High-purity quartzite is used to produce silicon and silica glass.
Geological facts
Quartzite is so tough that it often survives multiple cycles of erosion, outlasting softer rocks. When struck with a hammer, quartzite typically breaks through the quartz grains rather than around them, distinguishing it from sandstone.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and lack of foliation. Commonly found in riverbeds, glacial deposits, and along coastlines. It often has a slightly 'sugary' texture on fresh surfaces but appears smooth and waxy when water-worn.
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