igneous
Rounded dark stone, likely basalt or weathered mafic rock
Basalt (probable; mineral assemblage commonly plagioclase, pyroxene, and minor olivine)
AI-generated identification · may be incorrectDark brown-black, fine-grained, dull to slightly earthy luster, rounded by weathering or stream abrasion; estimated Mohs hardness 5–6, with no visible cleavage or crystals. The surface appears naturally weathered rather than polished.
Identified More igneous →
Explore Rounded dark stone, likely basalt or weathered mafic rock in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark brown-black, fine-grained, dull to slightly earthy luster, rounded by weathering or stream abrasion; estimated Mohs hardness 5–6, with no visible cleavage or crystals. The surface appears naturally weathered rather than polished.
Formation & geological history
Likely volcanic basalt formed from rapidly cooled mafic lava, generally Cenozoic or older; the exact age and source cannot be determined from the photograph. Its smooth shape probably reflects transport in water or prolonged erosion.
Uses & applications
Common basalt is used as crushed aggregate, road stone, railway ballast, riprap, and building stone; this small specimen has little commercial or jewelry value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the oceanic crust. Dark weathering coatings can obscure its original gray-black color.
Field identification & locations
Identify it by its high density, dark fine-grained appearance, and lack of obvious sedimentary layers; a streak, magnet test, fresh break, and hand lens would help distinguish it from chert or iron-rich metamorphic rock. Common worldwide, especially volcanic regions and river gravels.
