igneous
Weathered basalt cobble
Basalt; predominantly plagioclase feldspar, pyroxene, and often olivine
AI-generated identification · may be incorrectHardness about 5–6 Mohs; dark brown to gray, fine-grained, and typically dull to weakly vitreous. The specimen appears dense and massive, with no obvious cleavage; specific gravity is approximately 2.8–3.0.
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Physical properties
Hardness about 5–6 Mohs; dark brown to gray, fine-grained, and typically dull to weakly vitreous. The specimen appears dense and massive, with no obvious cleavage; specific gravity is approximately 2.8–3.0.
Formation & geological history
Basalt forms when low-silica magma erupts and cools rapidly at or near Earth’s surface, commonly in oceanic crust and volcanic fields. This beach cobble has been rounded and oxidized by wave abrasion and weathering; its exact age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive polished pieces may be collected but are rarely gemstone-grade.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the ocean floor. Brown coloration commonly reflects iron oxidation during weathering.
Field identification & locations
Identify it by its dark, heavy, fine-grained texture, rounded beach form, and lack of visible large crystals; a streak is usually gray to dark gray. Common along volcanic coastlines and glacially transported beaches.
