igneous
Dark igneous rock, likely basalt
Basalt; fine-grained mafic rock dominated by plagioclase and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to black, dull to weakly vitreous luster, fine-grained interlocking crystals, and poor cleavage as a whole rock. The specimen appears weathered and locally rusty, with lighter mineral patches; exact identification is limited by the image.
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Physical properties
Typically Mohs 5–6, dark gray to black, dull to weakly vitreous luster, fine-grained interlocking crystals, and poor cleavage as a whole rock. The specimen appears weathered and locally rusty, with lighter mineral patches; exact identification is limited by the image.
Formation & geological history
Forms when mafic lava cools rapidly at or near Earth’s surface, commonly during Cenozoic volcanic activity or older basaltic eruptions. It may weather to brown iron-oxide coatings and clay minerals.
Uses & applications
Widely used as crushed aggregate, road base, railroad ballast, riprap, and dimension stone; attractive polished pieces are used decoratively. Ordinary hand specimens have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms pillow lavas, lava flows, and columnar joints. Vesicles, if present, may later fill with zeolite, calcite, or quartz.
Field identification & locations
Look for a uniformly dark, dense, fine-grained texture and a greenish-black or rusty weathering rind; a streak test is usually gray. Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, and oceanic islands.
