igneous
Polished basalt pebble
Basalt, a fine-grained mafic volcanic rock; likely containing plagioclase, clinopyroxene, and iron-magnesium minerals
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to black, fine-grained, matte to subtly waxy when polished, with minor pale weathering or mineral patches. Its dense, massive texture lacks visible cleavage and has specific gravity around 2.8–3.1.
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Physical properties
Typically Mohs 5–6, dark gray to black, fine-grained, matte to subtly waxy when polished, with minor pale weathering or mineral patches. Its dense, massive texture lacks visible cleavage and has specific gravity around 2.8–3.1.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows and oceanic crust. Most examples are geologically young relative to continental rocks, ranging from recent eruptions to Archean specimens over 2.5 billion years old.
Uses & applications
Used as aggregate, roadstone, building stone, and occasionally as a polished decorative or landscaping stone; generally little jewelry value.
Geological facts
The rounded shape indicates water or sediment abrasion, while the smooth surface appears naturally worn or deliberately polished. Pale patches may be weathered feldspar, alteration minerals, or adhered sediment.
Field identification & locations
Identify it by its black, dense, fine-grained appearance, lack of layering, and inability to scratch with a fingernail; it may weakly attract a magnet if iron-rich. Common worldwide, especially near volcanic regions and beaches or riverbeds.
