metamorphic
Quartzite
Quartzite (Primary mineral: Silicon Dioxide, SiO2)
Hardness: 7 on Mohs scale; Color: typically white to gray, often with pinkish hues from iron oxide; Luster: vitreous to dull; Crystal structure: Non-foliated, interlocking quartz crystals; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.
- Hardness
- 7 on Mohs scale
- Luster
- vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white to gray, often with pinkish hues from iron oxide; Luster: vitreous to dull; Crystal structure: Non-foliated, interlocking quartz crystals; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.
Formation & geological history
Formed through the regional or contact metamorphism of quartz sandstone. Under intense heat and pressure, original quartz grains recrystallize into a dense, hard mosaic. Found in deformed mountain belts globally.
Uses & applications
Used widely in road construction (ballast), as a decorative building stone for countertops and tiles, in glassmaking if pure, and as a source of industrial silica.
Geological facts
Quartzite is so tough that it often outlasts the mountain ranges in which it forms, leaving behind 'monadnocks' or resistant ridges. It is often confused with marble, but marble will react to acid while quartzite will not.
Field identification & locations
Identify it in the field by its extreme hardness (grains won't rub off like sandstone and it will scratch glass) and its sugary appearance on fresh surfaces. Common in the Appalachian Mountains and Canadian Shield. Collectors look for pieces with unique stains or banding.
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