metamorphic
Quartzite Cobble
Metamorphosed Quartz Sandstone (SiO2)
Hardness: 7 (Mohs scale); Color: Tan, beige, or off-white with grainier textures; Luster: Dull to vitreous; Crystal structure: Trigonal/Hexagonal; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 (Mohs scale)
- Color
- Tan, beige, or off-white with grainier textures
- Luster
- Dull to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, beige, or off-white with grainier textures; Luster: Dull to vitreous; Crystal structure: Trigonal/Hexagonal; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed when quartz-rich sandstone is subjected to high heat and pressure, causing the original quartz grains to recrystallize and fuse into a solid rock. Typically found in mountain belts and areas of ancient continental crust.
Uses & applications
Used extensively in road construction, as railroad ballast, in the production of glass and industrial abrasives, and as decorative gravel.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rock in weathering processes, often forming the caps of ridges and mountains. It is harder than a steel knife and will easily scratch glass.
Field identification & locations
Identify in the field by its high hardness (it will scratch glass) and its sugary, granular texture. Unlike sandstone, when quartzite breaks, it breaks through the quartz grains rather than around them. Common in riverbeds and glacial deposits.
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