metamorphic
Quartzite
Quartzite (principally SiO2)
Hardness: 7 (Mohs scale); Color: Gray, tan, or white; Luster: Vitreous to dull; Crystal Structure: Trigonal (microcrystalline); Cleavage: None, fractures conchoidally; Specific Gravity: 2.6-2.7.
- Hardness
- 7 (Mohs scale)
- Color
- Gray, tan, or white
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, tan, or white; Luster: Vitreous to dull; Crystal Structure: Trigonal (microcrystalline); Cleavage: None, fractures conchoidally; Specific Gravity: 2.6-2.7.
Formation & geological history
Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure cause the sand grains to recrystallize and fuse together, creating a dense, interlocking matrix. Usually associated with orogenic (mountain-building) zones.
Uses & applications
Used extensively in road construction, railroad ballast, and as a decorative building stone. In the past, it was used by ancient civilizations for stone tools due to its extreme hardness and predictable fracture patterns.
Geological facts
Quartzite is so hard that it will scratch glass and even high-quality steel. Unlike sandstone, which breaks around its individual grains, quartzite is so well-fused that it breaks through the grains when struck.
Field identification & locations
Identify in the field by its extreme hardness and sugary, crystalline appearance on fresh surfaces. It is commonly found in ancient mountain ranges like the Appalachians or the Scottish Highlands. Commonly found as rounded river pebbles.
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