metamorphic
Quartzite (River Cobble)
Metamorphosed sandstone (SiO2)
Hardness: 7 on Mohs scale; Color: Grayish-white with iron-oxide staining (yellow/brown); Luster: Vitreous to waxy; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture)
- Hardness
- 7 on Mohs scale
- Color
- Grayish-white with iron-oxide staining (yellow/brown)
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Grayish-white with iron-oxide staining (yellow/brown); Luster: Vitreous to waxy; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture)
Formation & geological history
Formed when quartz-rich sandstone is subjected to intense heat and pressure, causing the individual quartz grains to recrystallize and fuse together. This specimen has been rounded by fluvial action in a river environment.
Uses & applications
Used as aggregate in construction, as a decorative stone in landscaping, and in glassmaking if pure enough. Historic use as a material for stone tools due to its hardness and sharpness when fractured.
Geological facts
Quartzite is so tough that it usually breaks through the quartz grains rather than along their boundaries, unlike sandstone. It is one of the most weather-resistant rocks on Earth.
Field identification & locations
Identify by its inability to be scratched by steel, its grainy sand-like texture visible under a lens, and its lack of reaction to acid. Commonly found in riverbeds and mountainous glacial deposits.
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