igneous
Dark basalt cobble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, very fine-grained, massive, and dull to slightly vitreous; Mohs hardness about 5–6, no visible cleavage, and specific gravity roughly 2.8–3.0. The rounded shape indicates water or abrasion wear rather than crystal form.
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Physical properties
Typically dark gray to black, very fine-grained, massive, and dull to slightly vitreous; Mohs hardness about 5–6, no visible cleavage, and specific gravity roughly 2.8–3.0. The rounded shape indicates water or abrasion wear rather than crystal form.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Its exact age cannot be determined visually; basalt ranges from recent to billions of years old.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and dimension stone; polished basalt is occasionally used for landscaping and decorative objects. Ordinary cobbles have little commercial jewelry value.
Geological facts
The specimen appears compact and vesicle-poor, suggesting relatively dense lava. A magnet may show weak attraction if magnetite is present, and a fresh broken surface is more diagnostic than its weathered exterior.
Field identification & locations
Common in volcanic and glacial regions, riverbeds, beaches, and landscaping gravel. Confirm with a hand lens for tiny plagioclase crystals and pyroxene, and distinguish it from andesite by its generally darker color and higher density.
