igneous
Rounded basalt pebble
Basalt, a fine-grained mafic volcanic rock composed chiefly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to faintly silky surface; typically massive with no visible cleavage, Mohs hardness about 5–6, and specific gravity roughly 2.8–3.0. Its smooth shape indicates water or wave abrasion, and the surface may be weathered or polished.
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Physical properties
Dark gray-brown, fine-grained, dull to faintly silky surface; typically massive with no visible cleavage, Mohs hardness about 5–6, and specific gravity roughly 2.8–3.0. Its smooth shape indicates water or wave abrasion, and the surface may be weathered or polished.
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. Individual pieces may range from recent volcanic material to rocks millions or billions of years old; age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, building stone, and occasionally as a polished decorative or landscaping stone; generally has little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It commonly attracts a weak magnet if it contains magnetite.
Field identification & locations
Identify it by its uniformly dark color, fine texture, substantial weight, and inability to scratch with a fingernail; it may scratch glass weakly. Common in streambeds, beaches, volcanic regions, and glacial deposits; avoid confusing it with dark slag or fine-grained gabbro.
