igneous
Rounded river pebble (likely weathered basalt or andesite)
Fine-grained mafic to intermediate volcanic rock; exact identification requires testing or a fresh surface
AI-generated identification · may be incorrectDark gray to brown, dull to weakly satin luster, fine-grained and apparently massive, with rounded abrasion-worn form; likely Mohs hardness about 5–6 and specific gravity roughly 2.7–3.1. The pale patch may be weathering, alteration, or a different mineral-rich surface zone.
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Physical properties
Dark gray to brown, dull to weakly satin luster, fine-grained and apparently massive, with rounded abrasion-worn form; likely Mohs hardness about 5–6 and specific gravity roughly 2.7–3.1. The pale patch may be weathering, alteration, or a different mineral-rich surface zone.
Formation & geological history
Probably an eroded volcanic rock fragment transported and rounded by running water or shoreline action; its geological age cannot be determined from the photograph. Basalt commonly forms from rapidly cooled lava, while andesite forms in volcanic arcs.
Uses & applications
Usually a landscaping, craft, or teaching pebble; unexceptional pieces have little commercial value and are not typically used as aggregate unless collected in bulk.
Geological facts
Basalt is commonly dominated by plagioclase and pyroxene, sometimes with olivine; andesite is dominated by plagioclase with pyroxene or amphibole. A streak test, magnet test, and fresh broken surface would improve identification.
Field identification & locations
Look for a dense, fine-grained texture, no visible layering, and possible weak magnetism for basalt; a glassy or vesicular texture would also support volcanic origin. Common in streambeds and beaches near volcanic or basaltic terrains.
