metamorphic
River Pebble (Gneiss or Quartzite variant)
Course-grained Metamorphic Rock (likely SiO2 dominant)
Hardness: 6-7 (Mohs scale); Color: Mottled grey, tan, and brown; Luster: Dull/Earthy (weathered surface); Structure: Crystalline but water-worn; Cleavage: None visible (conchoidal fracture likely if broken).
- Hardness
- 6-7 (Mohs scale)
- Color
- Mottled grey, tan, and brown
- Luster
- Dull/Earthy (weathered surface)
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Mottled grey, tan, and brown; Luster: Dull/Earthy (weathered surface); Structure: Crystalline but water-worn; Cleavage: None visible (conchoidal fracture likely if broken).
Formation & geological history
Formed through high-pressure metamorphism of granite or sandstone, followed by significant hydraulic erosion in a river or coastal environment, rounding the edges over thousands of years.
Uses & applications
Used primarily in landscaping, decorative ground cover, aquarium substrates, and as aggregate in construction and concrete manufacturing.
Geological facts
River pebbles are 'geological travelers'; their smooth shape is the result of 'abrasion'—thousands of collisions with other rocks during transport by water currents.
Field identification & locations
Identify by its smooth, ellipsoidal shape and absence of sharp edges. Commonly found in riverbeds, glacial till, and beaches. Collectors look for unique banding or internal translucent quartz veins.
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