igneous
Polished basalt pebble
Basalt; typically plagioclase feldspar, pyroxene, and minor olivine or magnetite
AI-generated identification · may be incorrectFine-grained, dark gray to black with brown weathering patches and a subdued vitreous-to-waxy luster; Mohs hardness about 6, no visible cleavage, and specific gravity roughly 2.8–3.0. The rounded, smooth surface indicates water abrasion or tumbling.
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Physical properties
Fine-grained, dark gray to black with brown weathering patches and a subdued vitreous-to-waxy luster; Mohs hardness about 6, no visible cleavage, and specific gravity roughly 2.8–3.0. The rounded, smooth surface indicates water abrasion or tumbling.
Formation & geological history
An extrusive mafic igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic episodes. This specimen was likely transported and rounded in a river, beach, or glacial deposit.
Uses & applications
Used as crushed aggregate, road stone, riprap, and architectural stone; attractive polished pebbles are used in landscaping, crafts, and collections.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms from mid-ocean-ridge or hotspot volcanism.
Field identification & locations
Identify it by its dense black color, fine texture, lack of obvious crystals, and strong resistance to scratching with a steel knife. Common worldwide, especially in volcanic regions and stream gravels.
