igneous
Rounded dark river cobble, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to weakly submetallic luster, massive texture, and no obvious cleavage; hardness about 5–6 Mohs, specific gravity roughly 2.8–3.0. Its smooth, flattened shape indicates natural water abrasion or possibly tumbling.
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Physical properties
Dark gray to black, fine-grained, dull to weakly submetallic luster, massive texture, and no obvious cleavage; hardness about 5–6 Mohs, specific gravity roughly 2.8–3.0. Its smooth, flattened shape indicates natural water abrasion or possibly tumbling.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, flood-basalt provinces, and volcanic fields. The specimen’s age cannot be determined from the photograph and may range from very recent to hundreds of millions of years.
Uses & applications
Used as crushed aggregate, roadstone, railway ballast, riprap, paving stone, and decorative landscaping material; attractive polished pieces are used as ornaments and paperweights.
Geological facts
The pale vein-like marking may be a fracture filled by a light mineral such as calcite, quartz, or zeolite. Exact identification would benefit from a fresh surface, streak, magnetism test, and microscopic examination.
Field identification & locations
Common in volcanic regions and streambeds worldwide, especially Hawaii, Iceland, India, the Pacific Northwest, and oceanic islands. Basalt is typically heavier than similarly colored sedimentary rocks and does not fizz in dilute hydrochloric acid unless carbonate-filled veins are exposed.
