igneous
Rounded dark river pebble, likely basalt
Basalt (fine-grained mafic volcanic rock; identification tentative from appearance)
AI-generated identification · may be incorrectDark gray to black, smooth and water-worn, with dull to faintly silky luster and subtle lighter streaks. Typically Mohs 6–6.5, dense (specific gravity about 2.8–3.0), composed of microscopic interlocking crystals with no visible cleavage.
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Physical properties
Dark gray to black, smooth and water-worn, with dull to faintly silky luster and subtle lighter streaks. Typically Mohs 6–6.5, dense (specific gravity about 2.8–3.0), composed of microscopic interlocking crystals with no visible cleavage.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly from volcanic flows or oceanic crust. Its age may range from recent to billions of years; this specimen was likely rounded by stream or beach transport.
Uses & applications
Used as crushed aggregate, roadstone, railroad ballast, building stone, and decorative landscaping material; attractive polished pieces are collected but usually have little commercial value.
Geological facts
The smooth shape and dark color are characteristic of a transported pebble rather than a fresh lava fragment. Fine pale lines may represent weathering, veins, or alignment of minerals.
Field identification & locations
Test for basalt by its high density, hardness, and generally nonporous texture; a streak test is usually gray, and a magnet may show weak attraction if magnetite is present. Common worldwide, especially volcanic regions, rivers, and beaches.
