igneous
Likely basalt
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to weakly vitreous, with massive texture and no obvious cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The specimen may also be a fine-grained metamorphic mafic rock, such as metabasalt, but the image is insufficient for certainty.
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Physical properties
Dark gray to black, fine-grained, dull to weakly vitreous, with massive texture and no obvious cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The specimen may also be a fine-grained metamorphic mafic rock, such as metabasalt, but the image is insufficient for certainty.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. It ranges from ancient Precambrian rocks to modern eruptions.
Uses & applications
Used widely as crushed aggregate, roadstone, railroad ballast, riprap, building stone, and basalt fiber; attractive polished pieces may be sold as decorative stone.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the ocean floor. Vesicles, magnetic response, or tiny visible olivine crystals can help distinguish it from similar dark rocks.
Field identification & locations
Identify it by its black-gray color, dense fine grain, lack of obvious layering, and occasional weak attraction to a magnet; a streak test is usually gray. Common locations include Hawaii, Iceland, oceanic islands, and continental volcanic provinces.
