sedimentary
Gray Pebble (Graywacke / Basalt)
Graywacke (or fine-grained Basalt)
Hardness: 6-7 on Mohs scale; Color: Dull medium to dark gray with subtle speckling; Luster: Matte, earthy; Texture: Fine-grained, stream-rounded; Cleavage: None
- Hardness
- 6-7 on Mohs scale
- Color
- Dull medium to dark gray with subtle speckling
- Luster
- Matte, earthy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dull medium to dark gray with subtle speckling; Luster: Matte, earthy; Texture: Fine-grained, stream-rounded; Cleavage: None
Formation & geological history
Formed through sedimentary deposition of poorly sorted sand grains in underwater environments, or through cooling of mafic lava, subsequently rounded by stream or wave erosion.
Uses & applications
Commonly used as construction aggregate, landscaping gravel, or river rock for decorative gardens.
Geological facts
Water-worn rounded pebbles often travel dozens to hundreds of miles down river systems, undergoing mechanical weathering that polishes their edges into smooth, sub-angular to rounded shapes.
Field identification & locations
Identified by its uniform gray color, fine texture, and smooth, rounded water-worn shape. Found along riverbeds, beaches, and glacial gravel deposits worldwide.
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