metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on the Mohs scale. Color: Brownish-tan to reddish-grey with waxy, vitreous, or sugary luster. Cleavage: None (conchoidal or uneven fracture). Crystal structure: Trigonal (microcrystalline interlocked quartz grains). Specific gravity: ~2.65.
- Hardness
- 7 on the Mohs scale
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Physical properties
Hardness: 7 on the Mohs scale. Color: Brownish-tan to reddish-grey with waxy, vitreous, or sugary luster. Cleavage: None (conchoidal or uneven fracture). Crystal structure: Trigonal (microcrystalline interlocked quartz grains). Specific gravity: ~2.65.
Formation & geological history
Formed when quartz-rich sandstone undergoes intense heat and pressure during regional metamorphism, fusing individual quartz grains into a dense, hard crystalline rock.
Uses & applications
Used as crushed stone in road construction, railway ballast, building stone, decorative landscaping, and historically as a tool stone.
Geological facts
Because quartzite is composed almost entirely of quartz, it is extremely resistant to chemical and physical weathering. It often forms prominent ridges and hills as surrounding softer rocks erode away.
Field identification & locations
Look for a dense, hard pebble or rock that scratches glass easily and does not fizz in acid. Unlike sandstone, when quartzite breaks, it fractures through the quartz grains rather than around them. Found globally in beach gravels, riverbeds, and metamorphic mountain belts.
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