igneous
Basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, with dull to subvitreous luster; hardness about 5–6, massive texture, and no prominent cleavage. The specimen appears dense and partly weathered, though its exact mineralogy cannot be confirmed from the image alone.
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Physical properties
Typically dark gray to black, fine-grained, with dull to subvitreous luster; hardness about 5–6, massive texture, and no prominent cleavage. The specimen appears dense and partly weathered, though its exact mineralogy cannot be confirmed from the image alone.
Formation & geological history
Forms when mafic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and continental lava flows. Most examples range from Precambrian to Recent age.
Uses & applications
Widely used as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished pieces are used decoratively but are uncommon as gemstones.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the ocean floor. Vesicles, olivine crystals, or a fine crystalline groundmass can help distinguish it from other dark rocks.
Field identification & locations
Identify it by its black color, heavy feel, fine texture, and lack of visible large crystals; a streak is usually gray to black and it commonly attracts no magnet. Common localities include Hawaii, Iceland, India’s Deccan Traps, and oceanic volcanic regions.
