igneous
Dark rounded pebble, possibly basalt
Basalt (fine-grained mafic volcanic rock; identification tentative from photograph)
AI-generated identification · may be incorrectBlack to dark gray, dull to faintly vitreous luster, smoothly weathered and rounded; typically Mohs 5–6, composed mainly of plagioclase and pyroxene with possible olivine. Crystal grains are microscopic, so cleavage is generally absent at specimen scale; density is commonly about 2.8–3.0 g/cm³.
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Physical properties
Black to dark gray, dull to faintly vitreous luster, smoothly weathered and rounded; typically Mohs 5–6, composed mainly of plagioclase and pyroxene with possible olivine. Crystal grains are microscopic, so cleavage is generally absent at specimen scale; density is commonly about 2.8–3.0 g/cm³.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Such rocks range from very young to Precambrian in age; the image alone cannot establish age.
Uses & applications
Used as crushed aggregate, road stone, railroad ballast, riprap, and occasionally decorative stone; small ordinary pebbles have little jewelry or commercial value.
Geological facts
Its dark color reflects abundant iron- and magnesium-rich minerals. Vesicles, tiny crystals, or a weak magnetic response from magnetite may support a basalt identification.
Field identification & locations
Common in volcanic regions and streambeds worldwide; distinguish it from slag by its natural texture and lack of obvious glassy bubbles, and from a meteorite by testing streak, density, and magnetism. A hand lens and fresh fracture would improve identification.
