igneous
Conglomeratic basalt (rounded volcanic breccia/pebble)
Basaltic conglomerate or polymict volcanic breccia; predominantly plagioclase–pyroxene basalt with lighter felsic or weathered clasts
AI-generated identification · may be incorrectDark gray to black, fine-grained basaltic matrix containing rounded tan, cream, and gray clasts; typically dull to weakly vitreous, hardness about 5–6, no obvious cleavage, and specific gravity roughly 2.7–3.0. The rounded shape suggests natural abrasion, possibly as a river or beach cobble.
Identified More igneous →
Explore Conglomeratic basalt (rounded volcanic breccia/pebble) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray to black, fine-grained basaltic matrix containing rounded tan, cream, and gray clasts; typically dull to weakly vitreous, hardness about 5–6, no obvious cleavage, and specific gravity roughly 2.7–3.0. The rounded shape suggests natural abrasion, possibly as a river or beach cobble.
Formation & geological history
Formed from volcanic rock fragments that were transported and cemented, or from redeposited basaltic lava/breccia; age cannot be determined from the photograph and depends on locality. Such rocks commonly form in volcanic terrains and erosion channels.
Uses & applications
Used mainly as a decorative or educational specimen, landscaping stone, or aggregate; generally unsuitable for jewelry except as a polished cabochon or display stone.
Geological facts
The contrasting light clasts and dark matrix indicate multiple volcanic or altered rock components rather than a single uniform mineral. A close inspection of thin sections or a fresh surface would distinguish conglomerate from breccia and identify the clasts precisely.
Field identification & locations
Look for a dark, dense matrix enclosing visibly separate clasts; rounded clasts indicate conglomerate, while angular fragments indicate breccia. Common in volcanic regions worldwide, including Iceland, Japan, western North America, and ocean-island terrains.
