igneous
Dark volcanic rock, possibly basalt
Basalt; commonly composed of plagioclase, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectFine-grained, dark gray to brown, dull to slightly vitreous luster, typically massive with no visible cleavage; hardness about 6 Mohs and specific gravity approximately 2.8–3.0. The rough, vesicle-poor surface and dark color are consistent with basalt, though the image alone cannot confirm it.
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Physical properties
Fine-grained, dark gray to brown, dull to slightly vitreous luster, typically massive with no visible cleavage; hardness about 6 Mohs and specific gravity approximately 2.8–3.0. The rough, vesicle-poor surface and dark color are consistent with basalt, though the image alone cannot confirm it.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly from volcanic flows; specimens may range from geologically recent to hundreds of millions of years old.
Uses & applications
Widely used as crushed aggregate, road stone, riprap, railroad ballast, and dimension stone; attractive polished varieties are used decoratively but usually have low gem value.
Geological facts
Basalt is the most abundant volcanic rock in oceanic crust and forms much of the seafloor, including mid-ocean ridges and many volcanic islands.
Field identification & locations
Identify by its dark color, fine texture, heavy weight, and lack of large crystals; a streak test is usually gray, and acid produces no reaction. Common worldwide, especially Hawaii, Iceland, India, and oceanic regions.
