igneous
Weathered basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-brown, rounded, and weakly granular with a dull to faintly subvitreous luster; typically massive with no cleavage. Hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0.
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Physical properties
Dark gray-brown, rounded, and weakly granular with a dull to faintly subvitreous luster; typically massive with no cleavage. Hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0.
Formation & geological history
Formed when mafic lava rapidly cooled at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; its rounded shape and surface alteration indicate stream or beach transport and weathering.
Uses & applications
Used as crushed aggregate, road base, riprap, railway ballast, and occasionally ornamental stone; this small weathered pebble has minimal commercial or jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, pillow lavas, and volcanic plateaus.
Field identification & locations
Identify it by its dark color, heavy feel, fine texture, and lack of visible large crystals; a streak test is usually gray, and a magnet may show weak attraction if iron-bearing minerals are abundant. Common worldwide in volcanic and alluvial settings.
