metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, or light orange; Luster: Vitreous to dull; Crystal structure: Trigonal/Hexagonal (microcrystalline/interlocking); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, or light orange
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, or light orange; Luster: Vitreous to dull; Crystal structure: Trigonal/Hexagonal (microcrystalline/interlocking); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the regional or contact metamorphism of quartz-rich sandstone. Under heat and pressure, original quartz grains recrystallize and fuse into a dense, interlocking matrix. These specimens are often further rounded by water erosion in stream beds or glacial deposits.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and high-purity silica source for glass making. Polished versions are used in jewelry and home decor.
Geological facts
Quartzite is so durable that it often outlasts the mountain ranges from which it originates, remaining as resistant caps on hills or as cobbles in river systems. It is harder than steel and will easily scratch a glass plate.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its grainy, sugary texture when viewed closely, which looks like fused sand grains. Commonly found in areas with ancient folded mountain belts.
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