igneous
Basalt cobble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and clinopyroxene with possible olivine and magnetite
AI-generated identification · may be incorrectDark gray-black, dense, dull to faintly vitreous, and fine-grained; likely Mohs 5–6, with no visible cleavage or large crystals. The rounded, weathered surface may reflect water transport or human tumbling.
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Physical properties
Dark gray-black, dense, dull to faintly vitreous, and fine-grained; likely Mohs 5–6, with no visible cleavage or large crystals. The rounded, weathered surface may reflect water transport or human tumbling.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic flows and oceanic crust. Basalt ranges from Archean to Recent in age and is still forming today.
Uses & applications
Used as crushed aggregate, road stone, riprap, paving, and dimension stone; attractive polished pieces are sold as decorative specimens. Typical value is about $2–$20 for a common cobble, or $20–$75 if unusually shaped or polished.
Geological facts
Its dark color and high density distinguish it from most granite and sandstone. Some basalt contains vesicles, but this specimen appears notably massive and non-vesicular.
Field identification & locations
Common worldwide, especially in volcanic regions, oceanic islands, flood-basalt provinces, and river gravels. Confirm with a fresh edge: it should show a dark fine-grained interior, resist scratching by a steel knife, and often attract a weak magnet if magnetite-rich.
