metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 on Mohs scale; Color: white/gray with occasional tan staining; Luster: vitrious to dull; Crystal structure: Trigonal/Hexagonal (microcrystalline); Cleavage: none (conchoidal fracture); Specific gravity: 2.63-2.65
- Hardness
- 7 on Mohs scale
- Color
- white/gray with occasional tan staining
- Luster
- vitrious to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: white/gray with occasional tan staining; Luster: vitrious to dull; Crystal structure: Trigonal/Hexagonal (microcrystalline); Cleavage: none (conchoidal fracture); Specific gravity: 2.63-2.65
Formation & geological history
Formed through the metamorphism of quartz-rich sandstone. High heat and pressure cause the quartz grains to recrystallize and fuse together, often in orogenic belts during mountain building.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and high-purity silica source for glass making. Sometimes used for countertops or decorative garden stone.
Geological facts
Quartzite is so hard and chemically resistant that it often forms the caprock or ridges of prominent hills. It is harder than a steel knife and will easily scratch glass.
Field identification & locations
Identify by its extreme hardness and the fact that it breaks across the original sand grains rather than around them. Found globally in ancient metamorphic terrains. Common in North American ranges like the Appalachians.
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