igneous
Polished basaltic rock, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectFine-grained, dark gray to brown-black, with a dull to faintly vitreous luster; Mohs hardness about 6, no prominent cleavage, and specific gravity roughly 2.8–3.0. The specimen appears water-worn and polished, with lighter brown weathering and dark dense bands.
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Physical properties
Fine-grained, dark gray to brown-black, with a dull to faintly vitreous luster; Mohs hardness about 6, no prominent cleavage, and specific gravity roughly 2.8–3.0. The specimen appears water-worn and polished, with lighter brown weathering and dark dense bands.
Formation & geological history
Basalt forms when low-silica mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic fields and oceanic crust. It can range from recent eruptions to Precambrian rocks billions of years old; its exact age cannot be determined from the image.
Uses & applications
Used extensively as crushed aggregate, road stone, riprap, building stone, and dimension stone; attractive polished pieces are used as décor and inexpensive lapidary specimens.
Geological facts
Basalt is Earth’s most abundant volcanic rock and commonly records seafloor spreading and hotspot volcanism.
Field identification & locations
Identify by its dark, dense, fine-grained texture, high weight, lack of visible large crystals, and lack of reaction to dilute acid. Common sources include Hawaii, Iceland, India’s Deccan Plateau, the ocean floor, and many volcanic regions.
