metamorphic
Quartzite (River Cobble)
Quartzite (dominant mineral: Quatrz, SiO2)
Hardness: 7 on Mohs scale (scratches glass); Color: White, grey, with tan/orange iron staining; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale (scratches glass)
- Color
- White, grey, with tan/orange iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale (scratches glass); Color: White, grey, with tan/orange iron staining; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the metamorphism of quartz sandstone under intense heat and pressure. The specimen shown is a river-worn cobble, meaning it has been mechanically weathered and rounded by water transport over thousands of years.
Uses & applications
Commonly used in construction as road ballast, gravel, or crushed stone. Highly pure varieties are used to produce silica sand for glassmaking and industrial abrasives.
Geological facts
Quartzite is so durable that it often survives long after the mountains it formed in have eroded away. It is tougher than the sandstone from which it originated because the grains are recrystallized and fused together.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch a steel knife) and its grainy, 'sugary' texture on fresh breaks. This rounded shape is common in riverbeds and glacial tills globally.
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