igneous
Rounded dark river pebble, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray, matte to faintly dull luster, very fine-grained and massive, with no visible cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. Its smooth, rounded shape reflects water or beach abrasion rather than crystal form.
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Physical properties
Dark gray, matte to faintly dull luster, very fine-grained and massive, with no visible cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. Its smooth, rounded shape reflects water or beach abrasion rather than crystal form.
Formation & geological history
Basalt forms when low-silica mafic lava rapidly cools at or near Earth’s surface, commonly from oceanic volcanism, flood-basalt eruptions, or volcanic islands. The specimen’s exact age and provenance cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and occasionally as landscaping or decorative stone; generally has little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of Earth’s oceanic crust. A hand lens may reveal tiny plagioclase laths, pyroxene grains, or vesicles.
Field identification & locations
Identify it by its dense dark-gray color, fine texture, lack of obvious layering, and inability to scratch with a copper coin; it should scratch glass only weakly or variably. Common sources include volcanic regions, river gravels, beaches, and glacial deposits.
