igneous
Rounded gray pebble, likely basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray, dull to weakly subvitreous, compact and fine-grained with no visible crystals; typically Mohs 5–6, massive, and lacking obvious cleavage. The smooth shape indicates water abrasion or tumbling.
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Physical properties
Dark gray, dull to weakly subvitreous, compact and fine-grained with no visible crystals; typically Mohs 5–6, massive, and lacking obvious cleavage. The smooth shape indicates water abrasion or tumbling.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined from the image and may range from recent to Precambrian.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, dimension stone, and occasionally landscaping material; ordinary pebbles have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Vesicles, if present, may later fill with zeolite, calcite, or quartz.
Field identification & locations
Identify it by its dense dark-gray color, fine texture, and lack of obvious layering or large grains; a streak, hardness, and magnetic response can help distinguish it from weathered granite or metamorphic rocks. Common in streambeds, beaches, volcanic regions, and glacial deposits.
