igneous
Tumbled river pebble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; exact identification requires testing
AI-generated identification · may be incorrectDark gray-green, fine-grained, dull to weakly satin luster, rounded and naturally or mechanically smoothed. Estimated hardness 5.5–6.5 Mohs; no visible cleavage or crystals, with a pale vein/fracture filling and possible weathering rind.
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Physical properties
Dark gray-green, fine-grained, dull to weakly satin luster, rounded and naturally or mechanically smoothed. Estimated hardness 5.5–6.5 Mohs; no visible cleavage or crystals, with a pale vein/fracture filling and possible weathering rind.
Formation & geological history
Likely formed from cooled lava millions to billions of years ago, then rounded by stream or beach abrasion; the orange-brown band may reflect iron oxidation or altered material. The white marking is probably a mineral-filled fracture, commonly quartz or calcite.
Uses & applications
Used as landscaping stone, aggregate, road material, and decorative pebble; this specimen has modest lapidary or educational value rather than gemstone value.
Geological facts
Basalt commonly contains plagioclase, pyroxene, and sometimes olivine; andesite is richer in silica and commonly contains plagioclase and amphibole. The exact rock type cannot be confirmed from the photograph alone.
Field identification & locations
Look for a uniformly fine texture, heavy feel, dark color, and lack of foliation; a streak, magnet test, hardness test, and fresh broken surface would help distinguish basalt from andesite or altered sedimentary rock. Common in volcanic regions and gravel beds worldwide.
