metamorphic
River Cobble (Gneiss/Schist mix)
Foliated Metamorphic Rock (Gneissic/Schistose)
Hardness: 6-7 on Mohs scale; Color: Grey, tan, and brown with subtle greenish hues; Luster: Dull to slightly vitreous; Structure: Foliated with visible mineral banding; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Grey, tan, and brown with subtle greenish hues
- Luster
- Dull to slightly vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grey, tan, and brown with subtle greenish hues; Luster: Dull to slightly vitreous; Structure: Foliated with visible mineral banding; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed from the high-pressure and high-temperature metamorphism of igneous or sedimentary protoliths. This specific specimen has been rounded by fluvial processes (river erosion) over thousands of years.
Uses & applications
Primarily used as decorative landscaping stone, river rock for drainage, or in construction as aggregate. Low individual collector value unless displaying unique mineral inclusions.
Geological facts
The rounded shape indicates a long history of transport in water, where tumbling against other rocks smoothed its angular edges. This process is a key part of the sedimentary cycle even though the rock itself is metamorphic.
Field identification & locations
Identify in the field by its rounded 'potato' shape and the presence of parallel mineral layers (foliation) visible on the worn surface. Found worldwide in active or ancient riverbeds.
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