igneous
River Cobble (Basaltic/Andesitic)
Mafic to Intermediate Volcanic Rock
Hardness: 6-7 (Mohs), Color: Dark grey to greenish-black with weathered brown rind, Luster: Dull/Waxy (damp), Structure: Fine-grained/Aphanitic, Cleavage: None
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Physical properties
Hardness: 6-7 (Mohs), Color: Dark grey to greenish-black with weathered brown rind, Luster: Dull/Waxy (damp), Structure: Fine-grained/Aphanitic, Cleavage: None
Formation & geological history
Formed from cooled lava (volcanic activity), subsequently rounded by fluvial (river) erosion. Likely Cenozoic era if found in active riverbeds.
Uses & applications
Landscaping, construction aggregate, and as a natural specimen for lapidary tumbling.
Geological facts
The smooth texture is a result of years of water transport, and the pitting or holes are often 'vesicles' from gas bubbles trapped in the cooling lava.
Field identification & locations
Identified by its rounded shape, dark interior color, and typical orange-brown oxidation (rust) on the outer surface. Very common in mountainous river systems.
