igneous
Rounded dark pebble, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray-brown, dull to faintly subvitreous luster, fine-grained and compact; Mohs hardness about 6, with no visible cleavage and typical specific gravity of 2.8–3.0. Its smooth oval shape indicates water or sediment abrasion.
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Physical properties
Dark gray-brown, dull to faintly subvitreous luster, fine-grained and compact; Mohs hardness about 6, with no visible cleavage and typical specific gravity of 2.8–3.0. Its smooth oval shape indicates water or sediment abrasion.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. The specimen was likely rounded in a stream, beach, or glacial deposit.
Uses & applications
Used as aggregate, road stone, railway ballast, riprap, and dimension stone; attractive polished pieces may be used decoratively. Ordinary pebbles have little commercial value.
Geological facts
Basalt is the most widespread volcanic rock on oceanic crust and commonly forms pillow lavas underwater. This small specimen cannot be identified conclusively without tests or a fresh surface.
Field identification & locations
Look for dark color, high density, fine texture, and possible tiny plagioclase crystals; a streak test is usually unhelpful. Common worldwide, especially volcanic regions, river gravels, beaches, and glacial deposits.
