metamorphic
Quartzite (specifically a river-worn cobble)
Quartzite (primarily Silicon Dioxide, SiO2)
Hardness: 7 on Mohs scale (scratches glass/steel); Color: Greyish-white interior with a tan/brown weathered rind; Luster: Vitreous to sugary (saccharoidal) on fresh surfaces; Crystal Structure: Hexagonal (microcrystalline); Cleavage: Uneven/conchoidal fracture.
- Hardness
- 7 on Mohs scale (scratches glass/steel)
- Color
- Greyish-white interior with a tan/brown weathered rind
- Luster
- Vitreous to sugary (saccharoidal) on fresh surfaces
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Physical properties
Hardness: 7 on Mohs scale (scratches glass/steel); Color: Greyish-white interior with a tan/brown weathered rind; Luster: Vitreous to sugary (saccharoidal) on fresh surfaces; Crystal Structure: Hexagonal (microcrystalline); Cleavage: Uneven/conchoidal fracture.
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone. It undergoes deep burial, heat, and pressure (regional metamorphism), causing the original sand grains to recrystallize and fuse together. This specific specimen has been further shaped and polished by water transport in a river or alluvial environment.
Uses & applications
Used extensively in construction as road ballast, gravel, and dimension stone for flooring or walls. High-purity quartzite is used to produce silica sand for glassmaking and industrial silicon.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rock, forming resistant ridges or hills. It is often confused with marble, but can be distinguished by its hardness; quartzite will scratch steel, while marble (calcite) will not.
Field identification & locations
Identify in the field by its extreme hardness, sugary texture where broken, and the way it fractures across sand grains rather than around them. Commonly found in areas with ancient mountain belts or as rounded cobbles in riverbeds and beaches.
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