metamorphic
River Cobble (Quartzite or Gneiss)
Metamorphic crystalline rock (predominantly SiO2 based)
Hardness: 6-7 on Mohs scale; Color: Mottled grey, tan, and brown; Luster: Dull/earthy (wet or polished can be vitreous); Structure: Granular to slightly foliated; Cleavage: None (conchoidal or irregular fracture); Specific Gravity: 2.6-2.8
- Hardness
- 6-7 on Mohs scale
- Color
- Mottled grey, tan, and brown
- Luster
- Dull/earthy (wet or polished can be vitreous)
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Mottled grey, tan, and brown; Luster: Dull/earthy (wet or polished can be vitreous); Structure: Granular to slightly foliated; Cleavage: None (conchoidal or irregular fracture); Specific Gravity: 2.6-2.8
Formation & geological history
Formed through the metamorphism of quartz-rich sandstone (Quartzite) or high-grade metamorphism of granite/sedimentary rocks (Gneiss). The rounded shape is a results of fluvial erosion (water transport in a river or stream) over thousands of years.
Uses & applications
Commonly used in landscaping, construction aggregate, and as decorative river stones. Smooth specimens are often used in massage therapy or for rock painting hobbies.
Geological facts
Rocks like this are 'wanderers'; they can be carried hundreds of miles from their original mountain source by glacial movement or river currents. Their smooth surface is a natural record of thousands of collisions with other rocks in moving water.
Field identification & locations
Identify in the field by its rounded, 'potato-like' shape and lack of sharp edges. Found globally in riverbeds, glacial tills, and coastal beaches. Collectors should look for unique banding or mineral inclusions.
