igneous
Rounded dark pebble, likely basalt
Basalt; predominantly plagioclase feldspar and pyroxene, with possible olivine
AI-generated identification · may be incorrectFine-grained, dark gray to black, dull to weakly vitreous luster, massive texture, and no visible cleavage; basalt hardness is typically 5–6 Mohs and specific gravity about 2.8–3.0. The smooth shape indicates water or abrasion wear, though the image cannot confirm the identification.
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Physical properties
Fine-grained, dark gray to black, dull to weakly vitreous luster, massive texture, and no visible cleavage; basalt hardness is typically 5–6 Mohs and specific gravity about 2.8–3.0. The smooth shape indicates water or abrasion wear, though the image cannot confirm the identification.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows and oceanic crust; specimens may range from recent to Precambrian in age. This pebble was likely eroded from a basalt outcrop and transported by a stream or shore.
Uses & applications
Used as crushed aggregate, road stone, riprap, railway ballast, and building stone; attractive pieces may be collected or polished. It has little jewelry value unless it contains unusual crystals or becomes a distinctive decorative stone.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, tiny pale feldspar crystals, or green olivine grains would support the identification.
Field identification & locations
Look for a uniformly dark, dense, fine-grained surface and a gray streak; it should not fizz with dilute acid. Common in volcanic regions, river gravels, and beaches; a definitive ID requires hardness, streak, density, and microscopy or geochemical testing.
