igneous
Dark basalt or volcanic rock fragment
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine or iron-titanium oxides
AI-generated identification · may be incorrectFine-grained, black to dark gray, with a dull to weakly glassy luster; hardness about 5–6 Mohs, massive texture, and no prominent cleavage. The specimen appears weathered and may contain small vesicles or iron-stained surfaces.
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Physical properties
Fine-grained, black to dark gray, with a dull to weakly glassy luster; hardness about 5–6 Mohs, massive texture, and no prominent cleavage. The specimen appears weathered and may contain small vesicles or iron-stained surfaces.
Formation & geological history
Basalt forms when low-viscosity mafic lava rapidly cools at or near Earth’s surface, commonly at mid-ocean ridges, hotspots, and continental volcanic fields. Individual pieces may range from recent volcanic material to ancient rocks billions of years old.
Uses & applications
Widely used as crushed aggregate, road metal, railway ballast, building stone, and basalt fiber; attractive polished pieces have minor decorative and collector value.
Geological facts
Its dark color reflects abundant ferromagnesian minerals and iron-rich oxides. Basalt is the most common volcanic rock and makes up much of Earth’s oceanic crust.
Field identification & locations
Identify it by its heavy, dark, fine-grained appearance and lack of visible large crystals; a streak, magnetism, vesicles, and thin-section or chemical testing can distinguish it from slag or other dark rocks. Common worldwide, especially volcanic regions.
