igneous
Black basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically Mohs 5–6, black to dark gray, dull to weakly vitreous, and very fine-grained; crystals are usually too small to see, with no obvious cleavage. The smooth shape indicates water-worn or manually polished material; the surface may also be coated or weathered.
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Physical properties
Typically Mohs 5–6, black to dark gray, dull to weakly vitreous, and very fine-grained; crystals are usually too small to see, with no obvious cleavage. The smooth shape indicates water-worn or manually polished material; the surface may also be coated or weathered.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; its exact age cannot be determined visually. Common sources include oceanic crust, lava flows, and volcanic river or beach gravels.
Uses & applications
Used as crushed aggregate, road base, riprap, dimension stone, landscaping rock, and occasionally in polished ornaments. This ordinary specimen has little jewelry value.
Geological facts
Basalt is the most widespread volcanic rock and makes up much of oceanic crust. Some basalt contains vesicles, but none are clearly visible in this specimen.
Field identification & locations
Identify it by its uniform dark color, high density, fine texture, and lack of visible layered grains; a streak, magnetism, and fresh broken surface would help distinguish it from shale, slag, or coal. Common worldwide, especially near volcanic regions.
