igneous
Likely basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to subtly vitreous luster; hardness about 5–6, no obvious cleavage, and typically massive with specific gravity around 2.8–3.0. The specimen may show weathering or a thin glassy surface.
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Physical properties
Dark gray to black, fine-grained, dull to subtly vitreous luster; hardness about 5–6, no obvious cleavage, and typically massive with specific gravity around 2.8–3.0. The specimen may show weathering or a thin glassy surface.
Formation & geological history
Forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. Basalt ranges from very young volcanic material to rocks billions of years old.
Uses & applications
Widely used as crushed aggregate, road base, railway ballast, dimension stone, and occasionally as polished decorative stone; generally low jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and relatively low silica content.
Field identification & locations
Identify by its dark color, heavy feel, fine texture, and lack of visible large crystals; a streak test is usually gray and it should not fizz with dilute acid. Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, and oceanic crust.
