igneous
Dark rounded river cobble, likely basalt
Basalt (fine-grained mafic volcanic rock; commonly plagioclase + pyroxene ± olivine)
AI-generated identification · may be incorrectDark gray to black, dull to weakly submetallic, fine-grained and massive, with no visible cleavage; typically Mohs 5–6 and specific gravity about 2.8–3.1. The smooth rounded shape indicates prolonged water or sediment abrasion.
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Physical properties
Dark gray to black, dull to weakly submetallic, fine-grained and massive, with no visible cleavage; typically Mohs 5–6 and specific gravity about 2.8–3.1. The smooth rounded shape indicates prolonged water or sediment abrasion.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age and provenance cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and occasionally dimension stone or landscaping material; attractive polished pieces are collected but are rarely used as gemstones.
Geological facts
The pale flecks may be weathered plagioclase, quartz, or surface mineral coatings; visual identification alone is not definitive because dark diabase, andesite, or other rocks can look similar.
Field identification & locations
Likely basalt if it is uniformly fine-grained and very dense; test for weak magnetism, inspect a fresh fracture, and compare streak and hardness. Common in volcanic regions and streambeds worldwide.
