metamorphic
Gray Quartzite River Rock
Meta-quartzite (SiO2)
Hardness: 7 on Mohs scale; Color: Light gray; Luster: Dull to vitreous; Crystal structure: Non-foliated granoblastic; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Light gray
- Luster
- Dull to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Light gray; Luster: Dull to vitreous; Crystal structure: Non-foliated granoblastic; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the metamorphism of quartz sandstone under high heat and pressure, typically during tectonic plate movements. This specimen has been rounded by fluvial action (water erosion) in a river or shore environment.
Uses & applications
Used in construction as crushed stone for road gravel and rail ballast, as decorative landscaping rock, and in the manufacturing of glass and ceramics if high purity quartz is present.
Geological facts
Quartzite is so tough that it often outlasts the mountain ranges from which it originated, frequently being found as weather-resistant caps on ridges. It is often mistake for marble, but unlike marble, it will not fizz when exposed to acid.
Field identification & locations
Identify in the field by its extreme hardness (it will scratch glass) and its sugary, granular surface texture that looks like sandpaper but feels smooth to the touch once water-worn. Found globally in ancient mountain belts.
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