igneous
Dark rounded pebble, likely basalt or andesite
Fine-grained mafic volcanic rock; commonly plagioclase feldspar, pyroxene, and possibly olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, and dull to subtly vitreous; Mohs hardness about 5–6, with no visible cleavage and a specific gravity near 2.8–3.0. The photographed specimen is too small and low-resolution for definitive identification.
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Physical properties
Typically dark gray to black, fine-grained, and dull to subtly vitreous; Mohs hardness about 5–6, with no visible cleavage and a specific gravity near 2.8–3.0. The photographed specimen is too small and low-resolution for definitive identification.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, generally during the Cenozoic or younger, though volcanic rocks occur throughout geological time. Rounded edges indicate transport and abrasion by water or sediment.
Uses & applications
Basalt and andesite are widely used as aggregate, roadstone, paving, riprap, and building stone; attractive polished pieces may be sold as inexpensive curios or landscaping stones.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. A hand lens may reveal tiny crystals, vesicles, or occasional green olivine grains.
Field identification & locations
Likely found in streambeds, beaches, volcanic terrains, and gravel deposits. Confirm with a fresh surface, hand lens, streak test, and density; a definitive name requires hardness, magnetism, and thin-section or chemical testing.
