igneous
Rounded river pebble, likely basalt or weathered andesite
Fine-grained mafic volcanic rock; commonly basalt, approximately plagioclase–pyroxene ± olivine
AI-generated identification · may be incorrectDark brown to gray, dull earthy luster, compact and fine-grained, with no visible crystals or cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.1. Surface rounding and weathering produce the smooth triangular form and brown coating.
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Physical properties
Dark brown to gray, dull earthy luster, compact and fine-grained, with no visible crystals or cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.1. Surface rounding and weathering produce the smooth triangular form and brown coating.
Formation & geological history
Likely formed from rapidly cooled lava during the Cenozoic or older, then weathered and transported by running water; exact age and identity require locality and testing. The pale spots may be weathering products or small altered mineral grains.
Uses & applications
Common basalt is used as aggregate, roadstone, riprap, and occasionally decorative stone; this individual specimen has mainly educational or modest collecting value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly forms oceanic crust and lava flows. A streak, magnet test, and fresh-surface examination would help distinguish it from andesite or other dark rocks.
Field identification & locations
Look for a dense, dark, nonlayered rock with a fine texture; a fresh chip should remain gray-black, and weak magnetism may occur if magnetite is present. Common worldwide, especially in volcanic regions and streambeds.
