igneous
Dark rounded river pebble, likely basalt
Basalt (fine-grained mafic volcanic rock; commonly plagioclase, pyroxene, and olivine)
AI-generated identification · may be incorrectHardness about 5–6 Mohs; charcoal-gray to black, fine-grained, dull to subtly vitreous luster, with no visible cleavage. The smooth oval shape indicates water abrasion or tumbling, and the specimen appears dense and nonporous.
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Physical properties
Hardness about 5–6 Mohs; charcoal-gray to black, fine-grained, dull to subtly vitreous luster, with no visible cleavage. The smooth oval shape indicates water abrasion or tumbling, and the specimen appears dense and nonporous.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Its exact geological age cannot be determined from the photograph.
Uses & applications
Used widely as crushed aggregate, road stone, railway ballast, riprap, and building stone; attractive polished pieces are used decoratively. This small pebble has minimal commercial value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and relatively little quartz.
Field identification & locations
Likely basalt if it is uniformly dark, heavy for its size, and lacks visible grains; a streak test, magnet response, and close inspection may help distinguish it from fine-grained gabbro or other dark rocks. Common sources include riverbeds, beaches, volcanic regions, and landscaping gravel.
