igneous
Dark volcanic rock, likely basalt
Basalt; fine-grained mafic silicate rock, commonly containing plagioclase, pyroxene, and minor olivine
AI-generated identification · may be incorrectDark gray to black, dull to weakly vitreous luster, massive and fine-grained; Mohs hardness about 6, with no obvious cleavage. The visible cavity may be a vesicle or weathering void, though the surface appears worn or possibly shaped.
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Physical properties
Dark gray to black, dull to weakly vitreous luster, massive and fine-grained; Mohs hardness about 6, with no obvious cleavage. The visible cavity may be a vesicle or weathering void, though the surface appears worn or possibly shaped.
Formation & geological history
Forms when low-silica basaltic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and flood-basalt provinces. Individual pieces may be geologically young or millions to billions of years old.
Uses & applications
Used as aggregate, road stone, riprap, ballast, and building stone; attractive polished basalt is used decoratively. This small specimen has little commercial value unless provenance or unusual texture is established.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of oceanic crust. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify by its dark, dense, fine-grained texture and lack of visible large crystals; a magnet may show weak attraction if iron minerals are abundant. Common worldwide, especially volcanic regions and beaches; confirm with thin-section or chemical testing if precise identification matters.
