metamorphic
Quartzite
Quartzite (mostly SiO2)
AI-generated identification · may be incorrectHardness: 7 on Mohs scale; Color: Gray, tan, white with visible crystalline texture; Luster: Vitreous to dull; Crystal structure: Trigonal/Hexagonal crystals within a grainy matrix; Cleavage: None (conchoidal fracture through grains)
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, white with visible crystalline texture
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, white with visible crystalline texture; Luster: Vitreous to dull; Crystal structure: Trigonal/Hexagonal crystals within a grainy matrix; Cleavage: None (conchoidal fracture through grains)
Formation & geological history
Formed when quartz-rich sandstone is subjected to intense heat and pressure (metamorphism), causing individual quartz grains to recrystallize and fuse into a dense, hard rock. Typically Proterozoic to Paleozoic in age.
Uses & applications
Used extensively in road construction (aggregate), railroad ballast, decorative stone in landscaping, and formerly for making stone tools. High-purity quartzite is used in glass and silicon production.
Geological facts
Unlike sandstone, which breaks around its sand grains, quartzite is so strongly fused that it breaks directly through the grains. It is extremely resistant to chemical weathering and creates prominent ridges in mountain ranges.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and sugary, crystalline appearance on fresh surfaces. Often found in riverbeds as rounded, smooth cobbles like this specimen.
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