igneous
Rounded dark river pebble, likely basalt
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, commonly with minor olivine and iron oxides
AI-generated identification · may be incorrectHardness about 5–6 Mohs; black to dark gray, fine-grained, dull to subtly glassy, and massive with no visible cleavage. The smooth shape and slight sheen indicate natural water abrasion or tumbling.
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Physical properties
Hardness about 5–6 Mohs; black to dark gray, fine-grained, dull to subtly glassy, and massive with no visible cleavage. The smooth shape and slight sheen indicate natural water abrasion or tumbling.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. Its age cannot be determined from the image and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, roadstone, railroad ballast, riprap, and building stone; attractive polished pieces are sold as decorative pebbles, though this specimen appears primarily natural and ornamental.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Dark color results from abundant ferromagnesian minerals and iron-bearing oxides.
Field identification & locations
Likely basalt if it is dense, opaque, fine-grained, and does not fizz in dilute acid; a streak, magnet test, and fresh break can help distinguish it from dark chert or slag. Common around volcanic regions and streambeds downstream from basalt outcrops.
