igneous
Rounded dark river cobble
Likely basalt (fine-grained volcanic rock); exact identification is uncertain from the image alone
AI-generated identification · may be incorrectDark gray-brown, smooth and water-worn, with a dull to weakly waxy luster and no visible crystals. If basalt, hardness is about 6 on Mohs scale and specific gravity about 2.8–3.0; crystal structure is too fine-grained to see.
Identified More igneous →
Explore Rounded dark river cobble in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray-brown, smooth and water-worn, with a dull to weakly waxy luster and no visible crystals. If basalt, hardness is about 6 on Mohs scale and specific gravity about 2.8–3.0; crystal structure is too fine-grained to see.
Formation & geological history
A volcanic rock formed when mafic lava cooled rapidly; its rounded shape likely developed through prolonged abrasion in flowing water. Basalt forms across a wide range of geological ages.
Uses & applications
Basalt is used as crushed aggregate, road stone, and building stone; this cobble is mainly suitable for display or collecting.
Geological facts
Water-worn cobbles can become rounded through repeated collisions and abrasion during transport in streams or along beaches.
Field identification & locations
The smooth, rounded form indicates transport, but color and texture alone cannot confirm basalt; a fresh broken surface, hardness test, and streak or magnetism checks would help. River and beach gravels commonly contain basalt cobbles where volcanic rocks occur upstream or nearby.
