igneous
Polished basalt cobble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine and iron oxides
AI-generated identification · may be incorrectTypically dark gray to black, brown-weathered, fine-grained, massive, and weakly vitreous to dull; Mohs hardness about 5–6, with no visible cleavage and specific gravity around 2.8–3.0. The smooth surface appears naturally water-worn or polished.
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Physical properties
Typically dark gray to black, brown-weathered, fine-grained, massive, and weakly vitreous to dull; Mohs hardness about 5–6, with no visible cleavage and specific gravity around 2.8–3.0. The smooth surface appears naturally water-worn or polished.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. Basalts range from Archean to modern age; the specimen’s age and exact source cannot be determined visually.
Uses & applications
Used as crushed aggregate, road stone, building stone, ballast, and occasionally dimension stone; polished pieces are popular educational and decorative specimens. It has little gemstone value.
Geological facts
The brown and metallic-looking flecks may reflect weathered iron minerals, magnetite, or tiny olivine-rich grains. A magnet may show weak attraction if magnetite is abundant.
Field identification & locations
Common in volcanic regions and river gravels worldwide, including Hawaii, Iceland, India, the Pacific Northwest, and ocean beaches. Field identification relies on its dark color, fine texture, high density, and lack of obvious layering or large crystals.
