metamorphic
Riverstone (Quartzite or Siltstone cobble)
Quartzite (primarily SiO2)
Hardness: 7 (Mohs scale); Color: Gray with faint brown and blue banding; Luster: Dull to waxy; Texture: Fine-grained, smooth, and rounded from water erosion; Cleavage: None (conchoidal fracture when broken).
- Hardness
- 7 (Mohs scale)
- Color
- Gray with faint brown and blue banding
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray with faint brown and blue banding; Luster: Dull to waxy; Texture: Fine-grained, smooth, and rounded from water erosion; Cleavage: None (conchoidal fracture when broken).
Formation & geological history
Formed from the recrystallization of quartz-rich sandstone through heat and pressure (metamorphism). Usually found as cobbles in riverbeds or glacial deposits where water has tumbled the stone into a smooth shape over thousands of years.
Uses & applications
Used in landscaping, decorative river rock arrangements, construction aggregate, and occasionally for stone carving or meditation 'worry stones'.
Geological facts
Quartzite is so hard that it can scratch glass and steel. This specimen shows 'rhythmic bedding' or metamorphic banding, reflecting its original sedimentary layers.
Field identification & locations
Identify in the field by its extreme hardness and smooth, water-worn exterior. It will not react with acid (unlike limestone) and is much harder than typical sandstone. Common in glacial outwash plains and river valleys.
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