igneous
Polished basalt pebble
Basalt, a fine-grained mafic volcanic rock; likely containing plagioclase, pyroxene, olivine, and iron oxides
AI-generated identification · may be incorrectDark gray to black, locally brown-weathered, with a glassy-to-dull polished luster and scattered pale mineral grains. Typical hardness is about 6 on Mohs; massive, nonfoliated, with no visible cleavage and specific gravity roughly 2.8–3.0.
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Physical properties
Dark gray to black, locally brown-weathered, with a glassy-to-dull polished luster and scattered pale mineral grains. Typical hardness is about 6 on Mohs; massive, nonfoliated, with no visible cleavage and specific gravity roughly 2.8–3.0.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; the rounded shape reflects water or sediment abrasion and polishing.
Uses & applications
Used as aggregate, roadstone, riprap, paving stone, and decorative polished material; ordinary pebbles have modest jewelry and collecting value.
Geological facts
Its dark color reflects abundant iron- and magnesium-rich minerals. The visible pale specks are most consistent with plagioclase, while rusty patches likely represent oxidation of iron-bearing minerals.
Field identification & locations
Identify it by its heavy, dark, fine-grained appearance, lack of layering, and resistance to scratching with a steel knife; it is commonly found in volcanic regions, river gravels, beaches, and glacial deposits.
