metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: whitish-gray to tan; Luster: vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: none; Specific gravity: 2.63-2.65. This specimen shows a water-worn, rounded texture.
- Hardness
- 7 on Mohs scale
- Color
- whitish-gray to tan
- Luster
- vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: whitish-gray to tan; Luster: vitreous to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: none; Specific gravity: 2.63-2.65. This specimen shows a water-worn, rounded texture.
Formation & geological history
Formed when quartz-rich sandstone is subjected to high heat and pressure (regional metamorphism), causing the sand grains to recrystallize and fuse together into a dense, hard rock.
Uses & applications
Used as a decorative stone, in construction as crushed stone for road beds or railway ballast, and historically for making stone tools. Also used in the glass and ceramics industries if high-purity silica is present.
Geological facts
Quartzite is so hard and chemically resistant that it often forms the ridges and peaks of mountain ranges because it resists erosion better than the surrounding rock.
Field identification & locations
Identified in the field by its extreme hardness (cannot be scratched by a steel blade) and the way it breaks through quartz grains rather than around them. Common in folded mountain belts and as river-rolled pebbles.
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