igneous
Likely weathered basalt or iron-rich volcanic rock
Fine-grained mafic rock, probably basalt; exact identification requires testing
AI-generated identification · may be incorrectDark brown-black, rough and irregular with a dull to weakly vitreous luster; likely Mohs hardness 5–6, massive texture, no obvious cleavage, and relatively high density. The rusty brown areas may reflect iron-oxide weathering.
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Physical properties
Dark brown-black, rough and irregular with a dull to weakly vitreous luster; likely Mohs hardness 5–6, massive texture, no obvious cleavage, and relatively high density. The rusty brown areas may reflect iron-oxide weathering.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, producing tiny crystals; samples may range from recent lava to Precambrian material depending on locality. Surface oxidation and abrasion likely caused the brown coloration.
Uses & applications
Common basalt is used as aggregate, road stone, riprap, building stone, and occasionally as a decorative material; this small weathered fragment has little commercial value.
Geological facts
Basalt commonly contains plagioclase, pyroxene, and sometimes olivine or magnetite. A magnet test may detect magnetite, while a fresh broken surface is usually darker than the weathered exterior.
Field identification & locations
Look for a fine-grained, dense, nonlayered rock that does not fizz with dilute hydrochloric acid; basalt is widespread in volcanic regions and oceanic crust. A streak, magnetism, density, and close-up texture would improve identification.
