metamorphic
Quartzite Cobble
Quartzite (primarily SiO2)
Hardness: 7 on Mohs scale; Color: Tan to yellowish-brown due to iron staining; Luster: Vitreous to dull; Crystal structure: Non-foliated, interlocking quartz grains; Specific Gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Tan to yellowish-brown due to iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan to yellowish-brown due to iron staining; Luster: Vitreous to dull; Crystal structure: Non-foliated, interlocking quartz grains; Specific Gravity: 2.6-2.7
Formation & geological history
Formed from quartz-rich sandstone through intense heat and pressure (metamorphism) that causes individual quartz grains to recrystallize into a dense, hard rock. This specimen has been rounded by fluvial or glacial transport.
Uses & applications
Used as aggregate in construction, road ballast, decorative garden stone, and as a refractory material in glass and metallurgical industries.
Geological facts
Quartzite is so tough that it often survives thousands of miles of travel in rivers and ocean surf where other rocks break down into sand. It is often harder than the steel blades used to cut it.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its grainy, sugary appearance on fresh surfaces. Often found as rounded 'river rocks' in creek beds or glacial till.
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