igneous
Weathered gray pebble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; probable basalt (plagioclase ± pyroxene/olivine)
AI-generated identification · may be incorrectDark gray, dull to faintly satin luster, rounded and naturally weathered; hardness about 5–6 Mohs, with no visible cleavage or crystals. Small orange-brown patches may be iron-oxide staining or weathered mafic minerals.
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Physical properties
Dark gray, dull to faintly satin luster, rounded and naturally weathered; hardness about 5–6 Mohs, with no visible cleavage or crystals. Small orange-brown patches may be iron-oxide staining or weathered mafic minerals.
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, likely millions to billions of years ago depending on its source terrain; subsequent erosion rounded it into a pebble. Exact identification requires a fresh surface, streak, density, or microscopy.
Uses & applications
Common basalt is used as crushed aggregate, roadstone, building stone, and occasionally landscaping material; ordinary pebbles have little commercial value.
Geological facts
Basalt is Earth’s most widespread volcanic rock and makes up much of the oceanic crust. Its fine texture reflects rapid cooling that prevented large crystals from developing.
Field identification & locations
Look for a dense, fine-grained dark rock that does not fizz with dilute acid and commonly attracts weakly to moderately to a magnet if iron-rich. Common worldwide, especially volcanic regions, river gravels, and beaches.
