igneous
Dark weathered igneous rock, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or iron oxides
AI-generated identification · may be incorrectFine-grained, dark gray to brown, dull to slightly submetallic luster, with rusty oxidation stains; typically hardness about 6, no obvious cleavage, and specific gravity around 2.8–3.0. The visible texture is consistent with dense, compact basalt, though the image alone cannot exclude a fine-grained mafic metamorphic r…
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Physical properties
Fine-grained, dark gray to brown, dull to slightly submetallic luster, with rusty oxidation stains; typically hardness about 6, no obvious cleavage, and specific gravity around 2.8–3.0. The visible texture is consistent with dense, compact basalt, though the image alone cannot exclude a fine-grained mafic metamorphic rock.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and flood-basalt provinces. The specimen’s weathering may have developed over thousands to millions of years; its exact age requires geological context.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, and dimension stone; attractive polished pieces are used decoratively but it is not generally a gemstone.
Geological facts
Fresh basalt commonly has tiny interlocking crystals invisible without magnification and may contain vesicles or magnetic iron minerals. Rust-colored patches usually reflect oxidation of iron-bearing minerals.
Field identification & locations
Common worldwide, especially Iceland, Hawaii, India’s Deccan Traps, ocean floors, and volcanic regions. A black, heavy, fine-grained stone that can scratch glass and may weakly attract a magnet supports basalt identification; value is usually $1–$10 for a hand specimen, or $10–$30 if distinctive or polished.
