metamorphic
Quartzite
Quartzite (primarily SiO2)
Hardness: 7 on Mohs scale; Color: white, grey, or yellowish; Luster: vitreous to dull; Crystal Structure: Trigonal (granular texture); Cleavage: none (conchoidal fracture); Specific Gravity: 2.6-2.7.
- Hardness
- 7 on Mohs scale
- Color
- white, grey, or yellowish
- Luster
- vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: white, grey, or yellowish; Luster: vitreous to dull; Crystal Structure: Trigonal (granular texture); Cleavage: none (conchoidal fracture); Specific Gravity: 2.6-2.7.
Formation & geological history
Quartzite is formed when quartz-rich sandstone is subjected to high heat and pressure (regional or contact metamorphism). This causes the original quartz grains to recrystallize into a dense, interlocking mosaic of crystals. It can date back over a billion years in some Precambrian shields.
Uses & applications
Used heavily in construction as road ballast, crushed stone, and decorative building stone or tiles. High-purity quartzite is used in the glass industry and for making silicon metal.
Geological facts
Quartzite is so hard that it will scratch glass and steel. In the field, it is often confused with marble, but unlike marble, it will not fizz when exposed to acid and cannot be scratched by a knife.
Field identification & locations
Identify by its extreme hardness and granular, sugary appearance. Found globally in mountainous regions associated with ancient tectonic activity. Collectors look for pieces with unique banding or inclusions.
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