igneous
Rounded volcanic cobble, likely basaltic andesite
Basaltic andesite (fine-grained volcanic rock; composition commonly plagioclase, pyroxene, and minor olivine or magnetite)
AI-generated identification · may be incorrectDark brown-gray, rounded and weathered, with a dull to faintly waxy luster and scattered pale mineral patches or vesicles. Typical hardness is about 6 on the Mohs scale; massive, fine-grained, with no visible cleavage and specific gravity around 2.7–3.0.
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Physical properties
Dark brown-gray, rounded and weathered, with a dull to faintly waxy luster and scattered pale mineral patches or vesicles. Typical hardness is about 6 on the Mohs scale; massive, fine-grained, with no visible cleavage and specific gravity around 2.7–3.0.
Formation & geological history
Formed when mafic to intermediate lava cooled rapidly at or near Earth’s surface, producing tiny crystals; the rounded shape indicates transport by water or glacial action. Its exact geological age cannot be determined from the photograph.
Uses & applications
Used locally as aggregate, road material, riprap, landscaping stone, and occasionally as a decorative cobble; generally not suitable for gemstone use.
Geological facts
Weathering can produce brown iron-oxide staining and accentuate pale feldspar crystals or amygdules. The specimen may also be ordinary basalt, and hand tests or a fresh broken surface would improve identification.
Field identification & locations
Common in volcanic terrains and streambeds worldwide, including the Cascades, Hawaii, Iceland, India, and oceanic islands. Look for a fine-grained dark matrix, high density, lack of layering, and resistance to scratching with a steel knife.
