igneous
Dark fine-grained river pebble, likely basalt or diabase
Basaltic rock; chiefly plagioclase feldspar and pyroxene, with possible magnetite
AI-generated identification · may be incorrectTypically Mohs 6–7, charcoal gray to black, fine-grained and massive, with a dull to weakly vitreous luster; the pale lines are likely calcite- or quartz-filled fractures. No obvious cleavage is visible.
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Physical properties
Typically Mohs 6–7, charcoal gray to black, fine-grained and massive, with a dull to weakly vitreous luster; the pale lines are likely calcite- or quartz-filled fractures. No obvious cleavage is visible.
Formation & geological history
Formed when mafic magma cooled rapidly at or near Earth’s surface; alteration and stream abrasion rounded the specimen. Basalt ranges from Precambrian to modern in age, depending on source.
Uses & applications
Used as crushed aggregate, roadstone, riprap, and occasionally as decorative or landscaping stone; this small pebble has minimal commercial value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly contains iron- and magnesium-rich minerals, giving it a dark color.
Field identification & locations
Identify it by its dark color, fine texture, high density, and lack of visible quartz or large crystals; a streak, magnetism test, and fresh break would distinguish basalt from slate or other dark rocks. Common in volcanic regions and river gravels worldwide.
