igneous
Likely weathered basalt or vesicular volcanic rock
Basalt, composed mainly of plagioclase feldspar, pyroxene, and minor olivine/iron oxides
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to weakly vitreous surface with small vesicles and pale weathering patches; Mohs hardness about 5–6, generally massive with no obvious cleavage, specific gravity roughly 2.8–3.1.
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Physical properties
Dark gray-brown, fine-grained, dull to weakly vitreous surface with small vesicles and pale weathering patches; Mohs hardness about 5–6, generally massive with no obvious cleavage, specific gravity roughly 2.8–3.1.
Formation & geological history
Forms when basaltic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows; specimens may be Cenozoic to Precambrian depending on locality, while surface weathering produces the brown coating.
Uses & applications
Used extensively as crushed aggregate, roadstone, railway ballast, riprap, and building stone; attractive polished pieces may be sold as inexpensive mineral specimens.
Geological facts
The rounded cavities are vesicles formed by escaping gas bubbles in lava; pale spots may be feldspar, zeolite, calcite, or weathering products, so exact identification requires testing.
Field identification & locations
Look for a dense dark fine-grained matrix, scattered gas cavities, and a streak that is usually gray rather than metallic. Common in volcanic regions worldwide, including Hawaii, Iceland, India, and the Pacific Northwest; a hand lens, streak test, and acid test can refine identification.
