metamorphic
Quartzite
Quartzite
Hardness: 7 (Mohs scale); Color: Gray, tan, or brown with iron staining; Luster: Vitreous to dull; Crystal structure: Trigonal (granular); Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7
- Hardness
- 7 (Mohs scale)
- Color
- Gray, tan, or brown with iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, tan, or brown with iron staining; Luster: Vitreous to dull; Crystal structure: Trigonal (granular); Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7
Formation & geological history
Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure cause the quartz grains to recrystallize and fuse together, creating a dense, interlocking matrix. Usually found in ancient mountain belts.
Uses & applications
Used as a decorative stone, road ballast, railway ballast, and in the manufacturing of glass and ceramics due to high silica content. Its extreme durability makes it ideal for construction stones.
Geological facts
Quartzite is so tough that it often outlasts the mountain ranges it formed in, remaining as resistant knobs or ridges. Unlike sandstone which breaks around grains, quartzite breaks through the grains.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel blade) and its grainy, sugary appearance. Found globally in areas with metamorphic basement rock. Collectors value well-rounded 'river cobble' forms like this for their smooth texture.
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