igneous
Weathered mafic rock, likely basalt
Basalt (fine-grained plagioclase–pyroxene rock; exact identification requires testing)
AI-generated identification · may be incorrectDark brown-gray, dull to weakly earthy luster, compact and fine-grained with weathering pits; likely hardness about 5–6 Mohs and no obvious cleavage. The rounded form suggests natural abrasion or weathering.
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Physical properties
Dark brown-gray, dull to weakly earthy luster, compact and fine-grained with weathering pits; likely hardness about 5–6 Mohs and no obvious cleavage. The rounded form suggests natural abrasion or weathering.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic fields and oceanic crust. Its age cannot be determined from the photograph; brown coloration reflects surface oxidation and weathering.
Uses & applications
Used as crushed aggregate, road stone, railway ballast, riprap, and occasionally building or landscaping stone; this small weathered piece has little commercial value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. Similar-looking weathered andesite, diabase, or metamorphic rocks cannot be excluded visually.
Field identification & locations
Look for a uniformly fine texture, dark streak, high density, and lack of visible quartz; a streak test, magnet response, fresh broken surface, and acid test can help distinguish it. Common sources include volcanic regions and gravel deposits.
