igneous
Polished dark river pebble, likely basalt
Basalt, a fine-grained mafic volcanic rock; possibly dolerite/diabase
AI-generated identification · may be incorrectTypically Mohs hardness 5–6, dark gray to black, fine-grained, with dull to faintly waxy luster and no visible cleavage. The smooth rounded shape indicates water abrasion or tumbling; pale streaks may be alteration or a mineral vein.
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Physical properties
Typically Mohs hardness 5–6, dark gray to black, fine-grained, with dull to faintly waxy luster and no visible cleavage. The smooth rounded shape indicates water abrasion or tumbling; pale streaks may be alteration or a mineral vein.
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. The specimen was likely transported and rounded in a river or beach environment; its exact age cannot be determined visually.
Uses & applications
Used as crushed aggregate, roadstone, railway ballast, riprap, and occasionally as polished decorative stone. This small piece has modest ornamental or educational value.
Geological facts
Basalt commonly contains plagioclase, pyroxene, and sometimes olivine or magnetite, giving it its dark color and relatively high density. A magnet may respond weakly if magnetite is abundant.
Field identification & locations
Identify it by its uniformly dark fine-grained texture, heavy feel, lack of obvious layering, and hardness sufficient to scratch glass. Common sources include volcanic regions and black-sand beaches; confirm with a streak, density, or thin-section test.
