igneous
Polished basalt pebble
Basalt, a fine-grained mafic volcanic rock dominated by plagioclase and pyroxene
AI-generated identification · may be incorrectTypically black to dark brown with a dull to submetallic polish; hardness about 6–6.5 Mohs, very fine-grained, massive, and lacking visible cleavage. The smooth rounded shape indicates water-worn or tumbled material; the bright streaks may reflect polishing or weathering.
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Physical properties
Typically black to dark brown with a dull to submetallic polish; hardness about 6–6.5 Mohs, very fine-grained, massive, and lacking visible cleavage. The smooth rounded shape indicates water-worn or tumbled material; the bright streaks may reflect polishing or weathering.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. Specimen age depends on locality, ranging from recent eruptions to Precambrian formations.
Uses & applications
Used as aggregate, road stone, riprap, paving, monuments, and decorative polished stone; attractive pieces are sold as worry stones or display specimens.
Geological facts
Its dark color reflects abundant ferromagnesian minerals and commonly magnetite. A streak test is usually gray to black, and some specimens are weakly magnetic.
Field identification & locations
Common in volcanic regions and river or beach gravels, especially Hawaii, Iceland, India, and oceanic islands. Confirm with high density, fine texture, lack of fizz in dilute acid, and possible magnetism; a photo alone cannot exclude another dark volcanic rock.
