igneous
Tumbled dark stone, likely basalt or basaltic pebble
Fine-grained mafic volcanic rock; commonly composed of plagioclase, pyroxene, and minor olivine (basalt)
AI-generated identification · may be incorrectDark brown-black, smoothly tumbled, opaque, and weakly glossy; likely Mohs hardness 5–6, with no visible cleavage and fine crystalline texture. The image alone cannot distinguish basalt from other dark rocks such as andesite, chert, or altered volcanic stone.
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Physical properties
Dark brown-black, smoothly tumbled, opaque, and weakly glossy; likely Mohs hardness 5–6, with no visible cleavage and fine crystalline texture. The image alone cannot distinguish basalt from other dark rocks such as andesite, chert, or altered volcanic stone.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and lava flows. Most examples range from recent volcanic ages to Precambrian rocks billions of years old.
Uses & applications
Used as aggregate, road stone, riprap, and building material; polished or tumbled pieces are inexpensive decorative and teaching specimens.
Geological facts
Its dark color reflects abundant ferromagnesian minerals and relatively low silica content. A streak, hardness, density, and close inspection of texture would improve identification.
Field identification & locations
Common worldwide, especially in volcanic regions and river gravels; it typically feels dense and does not fizz with dilute hydrochloric acid. Estimated value: $1–$5 for a common tumbled piece, or up to about $10–$20 if sold as a distinctive polished specimen.
