igneous
Dark fine-grained rock, likely basalt
Basalt; chiefly plagioclase feldspar and pyroxene, with possible olivine and iron-titanium oxides
AI-generated identification · may be incorrectHardness about 6–7 Mohs; dark gray to brown, dull to slightly subvitreous, massive and fine-grained. The scattered pale specks may be feldspar or weathered mineral grains; no obvious cleavage is visible.
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Physical properties
Hardness about 6–7 Mohs; dark gray to brown, dull to slightly subvitreous, massive and fine-grained. The scattered pale specks may be feldspar or weathered mineral grains; no obvious cleavage is visible.
Formation & geological history
Basalt forms when low-silica mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows and oceanic crust. Its exact age cannot be determined from the image and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, road stone, railway ballast, riprap, and occasionally building or landscaping stone; attractive polished pieces are collectible but generally inexpensive.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, magnetic response, and visible olivine or feldspar phenocrysts can help distinguish varieties.
Field identification & locations
Common worldwide, especially volcanic regions and oceanic islands. Test for a hard, non-acid-reactive surface and usually strong density; a streak, magnet test, and fresh broken surface would improve identification.
