metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, and light brownish-purple; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Granular texture.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, and light brownish-purple
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, and light brownish-purple; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Granular texture.
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone under high heat and pressure. The individual quartz grains recrystallize and fuse together, creating a dense, hard rock. It can date from almost any geological era.
Uses & applications
Used as a decorative stone in architecture, as crushed stone for road construction and railway ballast, and in the manufacturing of glass and ceramics due to high silica content.
Geological facts
Quartzite is so tough that it often survives erosion better than the surrounding rocks, frequently forming the peaks of ridges and mountains. Unlike sandstone, when quartzite breaks, it breaks through the quartz grains rather than around them.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. It is commonly found in fold mountain belts worldwide. Collectors look for unique colors or sugary textures.
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