igneous
Dark river-worn stone, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray to black, rounded and smoothly weathered, with a dull to slightly waxy luster. Typically Mohs 5–6, very fine-grained, massive, and without visible cleavage; exact identification is limited by the photograph.
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Physical properties
Dark gray to black, rounded and smoothly weathered, with a dull to slightly waxy luster. Typically Mohs 5–6, very fine-grained, massive, and without visible cleavage; exact identification is limited by the photograph.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. The specimen’s smooth shape indicates prolonged river or beach abrasion; its geologic age cannot be determined visually.
Uses & applications
Crushed basalt is used for road aggregate, concrete, railroad ballast, and construction stone; attractive polished pieces are used decoratively. This ordinary cobble has little jewelry or collector value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and relatively low silica content.
Field identification & locations
Look for a dense, dark, fine-grained texture, lack of layering, and no fizz with dilute hydrochloric acid; a magnet may show weak attraction if magnetite is present. Common worldwide, especially volcanic islands, ocean shores, and river gravels.
