igneous
Rounded dark volcanic rock, likely basalt
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, with possible olivine
AI-generated identification · may be incorrectDark gray-green, fine-grained, dull to faintly subvitreous luster; typically massive with no visible cleavage. Mohs hardness about 5–6 and specific gravity approximately 2.8–3.0; the rounded shape and surface weathering suggest a river or beach cobble.
Identified More igneous →
Explore Rounded dark volcanic rock, likely basalt in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray-green, fine-grained, dull to faintly subvitreous luster; typically massive with no visible cleavage. Mohs hardness about 5–6 and specific gravity approximately 2.8–3.0; the rounded shape and surface weathering suggest a river or beach cobble.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. Its smooth form was produced by prolonged abrasion in flowing water or surf.
Uses & applications
Used widely as crushed aggregate, road material, riprap, railroad ballast, and building stone; attractive pieces may be polished for décor, but this specimen has little gem value.
Geological facts
The curved surface lines may be fractures, weathering features, or remnants of flow texture rather than fossils. Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows and seafloor pillow lavas.
Field identification & locations
Identify by its uniformly fine texture, dark color, high density, and lack of obvious quartz or large crystals; a streak, magnetism, and a fresh broken surface would improve identification. Common in volcanic regions and transported river gravels.
