metamorphic
River Quartzite (or Quartz Cobble)
Quartzite (primarily Silicon Dioxide - SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brownish-yellow, and off-white due to iron oxide staining; Luster: Waxy to dull (weathered); Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Luster
- Waxy to dull (weathered)
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brownish-yellow, and off-white due to iron oxide staining; Luster: Waxy to dull (weathered); Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the regional metamorphism of quartz-rich sandstone. Under intense heat and pressure, the sand grains recrystallize and fuse into a dense rock. This specific specimen has been further shaped and smoothed by water transport in a river or creek environment.
Uses & applications
Used primarily as crushed stone for road construction, railroad ballast, and decorative landscaping. High-purity quartzite is used to produce glass and metallic silicon.
Geological facts
Quartzite is so tough that it often outlasts the surrounding rock layers, ending up as rounded cobbles in riverbeds. It is harder than a steel knife and will easily scratch glass.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by steel) and lack of visible grains compared to sandstone. Commonly found in riverbeds, glacial tills, and mountainous metamorphic belts worldwide.
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