Metamorphic
Quartzite
Quartzite (Primary mineral: SiO2)
Hardness: 7 on the Mohs scale. Color: White, gray, or tan (often with pinkish/brownish iron staining). Luster: Vitreous to dull. Crystal structure: Non-foliated, granular. Specific gravity: 2.6-2.7. Highly resistant to scratching and chemical weathering.
- Hardness
- 7 on the Mohs scale
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on the Mohs scale. Color: White, gray, or tan (often with pinkish/brownish iron staining). Luster: Vitreous to dull. Crystal structure: Non-foliated, granular. Specific gravity: 2.6-2.7. Highly resistant to scratching and chemical weathering.
Formation & geological history
Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure cause the individual sand grains to recrystallize and fuse together. This specimen appears to be a water-worn river cobble or 'glacial erratic' smoothed by transport.
Uses & applications
Used in construction as crushed stone for road beds, railway ballast, and roofing granules. High-quality slabs are used for laboratory countertops and decorative flooring. Also used in glass making and metallurgy as a source of silica.
Geological facts
Quartzite is so tough that when it breaks, it fractures through the quartz grains rather than along the boundaries between them, unlike its parent rock sandstone. It is often found capping mountain peaks because it resists erosion much better than surrounding rocks.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its grainy, 'sugary' appearance when viewed closely. Common in riverbeds, glacial deposits, and ancient mountain belts. For collectors, look for versions with unique colors or inclusions.
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