metamorphic
Quartzite
Quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: Gray with reddish-brown iron staining; Luster: Dull to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None; Specific Gravity: 2.63-2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray with reddish-brown iron staining
- Luster
- Dull to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray with reddish-brown iron staining; Luster: Dull to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None; Specific Gravity: 2.63-2.65.
Formation & geological history
Formed from the metamorphism of quartz sandstone under intense heat and pressure, typically during mountain-building events (orogeny). This resulting recrystallization interlocks the quartz grains into an extremely durable mass.
Uses & applications
Used in construction as road ballast, railway ballast, and crushed stone. High-purity quartzite is used to produce silica sand for glassmaking and silicon for electronics.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rocks during erosion, remaining as resistant ridges or 'monadnocks'. It is harder than a steel knife and will easily scratch glass.
Field identification & locations
Identify by its extreme hardness and sugary, crystalline texture on a fresh break. It is commonly found in ancient mountain belts like the Appalachians or the Scottish Highlands. Collectors look for unique colors or water-worn 'river cobble' shapes.
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