igneous
Rounded dark river cobble, likely basalt
Basalt; fine-grained mafic volcanic rock, commonly plagioclase + pyroxene ± olivine
AI-generated identification · may be incorrectDark gray to brown, fine-grained, dull to weakly waxy luster, with rounded water-worn surfaces and no obvious crystals or cleavage. Typical hardness is about 6 Mohs and specific gravity about 2.8–3.0; the pale speckling may be weathered feldspar or alteration minerals.
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Physical properties
Dark gray to brown, fine-grained, dull to weakly waxy luster, with rounded water-worn surfaces and no obvious crystals or cleavage. Typical hardness is about 6 Mohs and specific gravity about 2.8–3.0; the pale speckling may be weathered feldspar or alteration minerals.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly from oceanic volcanism or continental eruptions. The cobble’s rounding indicates later transport by a river or beach; its exact geological age cannot be determined from the photograph.
Uses & applications
Used as aggregate, road stone, riprap, paving stone, and occasionally landscaping or decorative material; this individual cobble has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the ocean floor. It can resemble fine-grained andesite, so a streak, density, and thin-section or chemical test may be needed for certainty.
Field identification & locations
Identify it by its heavy feel, dark fine-grained texture, lack of visible quartz, and strong resistance to scratching by steel. Common sources include volcanic regions and basaltic river gravels worldwide.
