igneous
Polished basalt pebble
Basalt, a fine-grained mafic volcanic rock composed chiefly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectTypically dark brown to black, fine-grained, dense, and nonmetallic with a dull to glassy polish; hardness about 5–6 Mohs, no visible cleavage, and specific gravity roughly 2.8–3.1. The rounded shape and smooth surface indicate natural water abrasion or tumbling.
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Physical properties
Typically dark brown to black, fine-grained, dense, and nonmetallic with a dull to glassy polish; hardness about 5–6 Mohs, no visible cleavage, and specific gravity roughly 2.8–3.1. The rounded shape and smooth surface indicate natural water abrasion or tumbling.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Individual specimens may range from recent volcanic material to rocks millions or billions of years old; age cannot be determined visually.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, building stone, and occasionally in landscaping or polished-stone collections. This small common pebble generally has little commercial value.
Geological facts
Tiny pale specks may be feldspar, vesicles, weathering products, or surface inclusions; the image alone cannot confirm the exact variety. A magnet may show weak attraction if the basalt contains abundant magnetite.
Field identification & locations
Identify it by its dark dense appearance, fine texture, and lack of distinct large crystals; a streak test, magnet test, and fresh broken surface help distinguish basalt from slag or dark sedimentary rock. Common in volcanic regions and stream gravels worldwide.
