igneous
Likely basalt fragment
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, with a dull to weakly vitreous luster and irregular fracture; Mohs hardness about 5–6, with no obvious cleavage visible. The specimen appears naturally angular and may show minor weathering or vesicles.
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Physical properties
Dark gray to black, fine-grained, with a dull to weakly vitreous luster and irregular fracture; Mohs hardness about 5–6, with no obvious cleavage visible. The specimen appears naturally angular and may show minor weathering or vesicles.
Formation & geological history
Basalt forms when low-viscosity mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Most examples range from recent volcanic material to hundreds of millions of years old; age cannot be determined from the photograph.
Uses & applications
Used extensively as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive polished pieces are sometimes collected but are generally not gemstone material.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its exact identification is tentative from an image alone because dark slag, gabbro, or other mafic rocks can look similar.
Field identification & locations
Look for a uniformly fine texture, high density, dark color, and resistance to scratching with a steel knife; a streak test is usually gray and non-diagnostic. Common worldwide, especially Hawaii, Iceland, India, and oceanic islands.
