metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, and yellowish-brown (due to iron oxidation); Luster: Vitreous to greasy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, and yellowish-brown (due to iron oxidation)
- Luster
- Vitreous to greasy
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, and yellowish-brown (due to iron oxidation); Luster: Vitreous to greasy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone. Under intense heat and pressure, quartz grains recrystallize and fuse together, creating a dense, hard rock. It is found in mountain belts globally, often dating from the Precambrian to more recent tectonic events.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and building stone. High-quality smooth specimens are used for lapidary work and decorative landscaping.
Geological facts
Quartzite is so hard and chemically resistant that it often forms the ridges and peaks of mountain ranges because it weathers much more slowly than surrounding rocks like marble or shale.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch glass and steel) and its crystalline, sugary appearance on freshly broken surfaces. Unlike sandstone, quartzite breaks across the quartz grains rather than around them.
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