metamorphic
Quartzite River Cobble
Quartzite (primarily SiO2)
Hardness: 7 on Mohs scale; Color: Tan, yellow-brown, and grey; Luster: Dull to waxy; Texture: Fine-grained, sugary (granoblastic); Cleavage: None; Specific Gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Tan, yellow-brown, and grey
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, yellow-brown, and grey; Luster: Dull to waxy; Texture: Fine-grained, sugary (granoblastic); Cleavage: None; Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the metamorphism of quartz-rich sandstone under intense heat and pressure, typically during mountain-building events. This specific specimen has been further shaped by fluvial (river) erosion, resulting in its rounded cobble shape.
Uses & applications
Used in construction as road ballast, decorative landscaping, and occasionally as a source of high-purity silica for industrial glass making.
Geological facts
Quartzite is so hard that when it breaks, it fractures through the individual quartz grains rather than around them, unlike its parent rock, sandstone. The yellowish staining is likely due to iron oxide deposits.
Field identification & locations
Identify in the field by its extreme hardness (it will easily scratch glass) and its smooth, rounded surface resulting from water transport. It is common in riverbeds and glacial till throughout North America.
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