igneous
Dark basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and often olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, massive, and with a dull to faintly vitreous luster; hardness about 5–6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0. The smooth edges suggest water-worn or tumbled material.
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Physical properties
Typically dark gray to black, fine-grained, massive, and with a dull to faintly vitreous luster; hardness about 5–6 Mohs, no obvious cleavage, and specific gravity roughly 2.8–3.0. The smooth edges suggest water-worn or tumbled material.
Formation & geological history
Forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and ancient lava flows; specimens may range from modern to billions of years old.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, building stone, and occasionally decorative landscaping material. Ordinary pebbles have little jewelry value.
Geological facts
Its dark color reflects abundant mafic minerals and relatively low silica content; tiny vesicles may occur, though none are clearly visible here.
Field identification & locations
Common around volcanic regions and river or beach gravels derived from basalt; identify by its uniform dark fine grain, substantial weight, and inability to scratch glass reliably. Exact identification would benefit from a fresh surface or tests for streak and density.
