igneous
Dark basalt or fine-grained mafic rock
Basalt, composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to weakly subvitreous luster; hardness about 5–6 Mohs, no obvious cleavage, and typically dense (specific gravity ~2.8–3.1). The image lacks enough detail to distinguish basalt confidently from other dark rocks.
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Physical properties
Dark gray to black, fine-grained, dull to weakly subvitreous luster; hardness about 5–6 Mohs, no obvious cleavage, and typically dense (specific gravity ~2.8–3.1). The image lacks enough detail to distinguish basalt confidently from other dark rocks.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and flood-basalt provinces. It may range from recent to Precambrian in age.
Uses & applications
Crushed basalt is widely used for road aggregate, concrete, railway ballast, riprap, and construction stone; attractive varieties are used decoratively but have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, magnetic response, or visible tiny crystals can help confirm it.
Field identification & locations
Look for a uniformly fine texture, dark color, heavy weight, and occasional small gas cavities; a streak test is usually gray and a magnet may respond weakly. Common worldwide, especially volcanic regions and river gravels.
