igneous
River Cobble (Basaltic/Andesitic)
Mafic Igneous Porphyritic Rock
Hardness (Mohs scale): 6-7; Color: Dark grey to black with brownish weathering spots; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained) with possible micro-phenocrysts; Cleavage: None (conchoidal fracture when broken); Specific gravity: 2.8 - 3.0
- Color
- Dark grey to black with brownish weathering spots
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness (Mohs scale): 6-7; Color: Dark grey to black with brownish weathering spots; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained) with possible micro-phenocrysts; Cleavage: None (conchoidal fracture when broken); Specific gravity: 2.8 - 3.0
Formation & geological history
Formed from the cooling of magnesium and iron-rich lava. This specific specimen’s rounded 'egg' shape was created through fluvial abrasion (water erosion) in a riverbed or coastline. Geological age is variable but commonly Cenozoic.
Uses & applications
Used in construction as aggregate, landscaping, decorative stone, or for grinding tools in ancient cultures. Generally used as a filler in roads or concrete.
Geological facts
Rocks like these are often called 'river stones.' The reddish-brown spots are likely 'weathering rinds' where iron-rich minerals are oxidizing (rusting) upon exposure to oxygen and water.
Field identification & locations
Identify by its heavy weight (density), smooth water-worn surface, and dark internal color. They are ubiquitous in river valleys and glaciated plains.
