metamorphic
Quartzite
Metamorphic Quartzite (predominantly SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, or white; Luster: Vitreous (glassy) to dull; Crystal structure: Non-foliated, granular; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or white
- Luster
- Vitreous (glassy) to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or white; Luster: Vitreous (glassy) to dull; Crystal structure: Non-foliated, granular; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the regional metamorphism of quartz sandstone. Intense heat and pressure cause the quartz grains to recrystallize and fuse together, creating an extremely durable rock. Prevalent in ancient mountain belts.
Uses & applications
Used widely in road construction (ballast), as decorative stone in landscaping, for kitchen countertops, and as industrial raw material for glass making and metallurgy.
Geological facts
Quartzite is so hard and chemically resistant that it often forms the caps of ridges and mountains because it resists erosion better than the surrounding rock. Unlike sandstone, quartzite fractures across the original sand grains rather than around them.
Field identification & locations
Identify by its ability to scratch glass and its grainy, 'sugary' texture on fresh breaks. It is found worldwide, notably in the Appalachian Mountains and the Lake Superior region. Collectors look for pieces with iron staining or unique inclusions.
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