igneous
Black volcanic rock, likely basalt
Basalt (fine-grained mafic silicate rock; chiefly plagioclase, pyroxene, and commonly olivine)
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to slightly vitreous luster; hardness about 5–6 Mohs, with no obvious cleavage and typically high density (about 2.8–3.0 g/cm³). The specimen appears angular and may contain pale plagioclase or weathered mineral patches.
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Physical properties
Dark gray to black, fine-grained, dull to slightly vitreous luster; hardness about 5–6 Mohs, with no obvious cleavage and typically high density (about 2.8–3.0 g/cm³). The specimen appears angular and may contain pale plagioclase or weathered mineral patches.
Formation & geological history
Forms when basaltic lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and continental lava flows; individual pieces may range from recent to billions of years old.
Uses & applications
Widely used as crushed aggregate, road stone, railroad ballast, building stone, and occasionally as a polished decorative or jewelry material.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify by its black, dense, fine texture and inability to scratch easily with a steel nail; a white streak or strong fizzing would suggest a different material. Common worldwide in volcanic regions, including Hawaii, Iceland, India, and the Pacific Northwest.
