metamorphic
Quartzite (River Cobble)
Quartzite (SiO2)
Hardness: 7 on Mohs scale (scratches glass); Color: Grayish-white with tan/orange iron staining; Luster: Vitreous to dull; Crystal structure: Granular/Microcrystalline; Cleavage: None (conchoidal to irregular fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale (scratches glass)
- Color
- Grayish-white with tan/orange iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale (scratches glass); Color: Grayish-white with tan/orange iron staining; Luster: Vitreous to dull; Crystal structure: Granular/Microcrystalline; Cleavage: None (conchoidal to irregular fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone under intense heat and pressure, causing sand grains to recrystallize into a dense, interlocking matrix. This specific specimen shows signs of water-wear (smooth edges), suggesting it was tumbled in a river or glacial environment.
Uses & applications
Used widely in construction as crushed stone or road ballast. Historically used for making stone tools (lithics) when high-grade flint was unavailable. In modern times, it is used for decorative landscaping and as a source of silica in glassmaking.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rock layers, forming prominent ridges and peaks. It is frequently mistaken for marble, but can be distinguished by its inability to be scratched by a steel knife and its lack of reaction to acid.
Field identification & locations
Identify in the field by its sugary, granular texture that looks like sand but cannot be rubbed off. It will easily scratch a glass bottle or steel blade. Commonly found in riverbeds, glacial tills, and ancient mountain belts. Collectors look for pieces with unique banding or iron-oxide staining.
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