igneous
Basalt pebble
Fine-grained mafic volcanic rock, typically plagioclase + clinopyroxene ± olivine and magnetite
AI-generated identification · may be incorrectDark gray-black, fine-grained, dull to slightly vitreous luster, massive with no visible cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The rounded, smooth form indicates water or abrasion polishing.
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Physical properties
Dark gray-black, fine-grained, dull to slightly vitreous luster, massive with no visible cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. The rounded, smooth form indicates water or abrasion polishing.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. Its age may range from recent eruptions to billions of years; the image alone cannot date it.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, building stone, and occasionally as polished decorative material or a massage stone.
Geological facts
Its black color can resemble slag, coal, or meteorite, but basalt commonly has tiny vesicles or crystalline texture and is usually weakly magnetic rather than strongly metallic.
Field identification & locations
Common along beaches, rivers, lava fields, and volcanic regions worldwide; inspect a fresh broken surface for fine interlocking crystals and test for magnetism. Typical value is $1–$15 for a common pebble, or $10–$40 if especially well polished or decorative.
