igneous
Dark mafic rock, likely basalt
Basalt; chiefly plagioclase feldspar, clinopyroxene, and commonly olivine or iron oxides
AI-generated identification · may be incorrectFine- to medium-grained, very dark gray to black, with a dull to faintly vitreous luster; hardness about 5–6 Mohs, massive texture, and no obvious cleavage. The specimen appears weathered or vesicle-poor, so exact identification from the image is tentative.
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Physical properties
Fine- to medium-grained, very dark gray to black, with a dull to faintly vitreous luster; hardness about 5–6 Mohs, massive texture, and no obvious cleavage. The specimen appears weathered or vesicle-poor, so exact identification from the image is tentative.
Formation & geological history
Basalt forms when low-viscosity mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and hotspots. It may range from ancient Precambrian rocks to modern eruptions.
Uses & applications
Used as crushed aggregate, road base, railway ballast, building stone, and sometimes polished decorative stone; ordinary specimens have little jewelry value.
Geological facts
Basalt is the most widespread volcanic rock and forms much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and iron-rich oxides.
Field identification & locations
Look for a dense black fine-grained texture, magnetic response in some examples, and possible tiny plagioclase crystals or vesicles; distinguish it from glassy obsidian by its crystalline texture. Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, and oceanic islands.
