igneous
Dark mafic river pebble, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectFine-grained, dark gray to black, with a dull to weakly vitreous luster; typically Mohs 5–6, massive, non-cleavable, and relatively dense (about 2.8–3.0 g/cm³). The white lines appear to be quartz or calcite-filled fractures/veins, though the photo is insufficient for certainty.
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Physical properties
Fine-grained, dark gray to black, with a dull to weakly vitreous luster; typically Mohs 5–6, massive, non-cleavable, and relatively dense (about 2.8–3.0 g/cm³). The white lines appear to be quartz or calcite-filled fractures/veins, though the photo is insufficient for certainty.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined visually; the rounded shape indicates water abrasion after formation.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, building stone, and landscaping material; attractive polished pieces may be sold as inexpensive specimens.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly forms oceanic crust. Magnetic response may occur if magnetite is present.
Field identification & locations
Identify by its dark, fine-grained, heavy appearance and lack of visible large crystals; test with a steel file and inspect a fresh surface, while dilute acid can distinguish calcite veins by fizzing. Common worldwide, especially near volcanic regions and streambeds.
