igneous
Likely basalt (possibly a dark beach pebble)
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray to black, dull to weakly waxy luster, massive and fine-grained; Mohs hardness about 5–6, with no visible cleavage and typically high density (about 2.8–3.1 g/cm³). Its smooth, rounded shape indicates water or abrasion, possibly with minor weathering or iron staining.
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Physical properties
Dark gray to black, dull to weakly waxy luster, massive and fine-grained; Mohs hardness about 5–6, with no visible cleavage and typically high density (about 2.8–3.1 g/cm³). Its smooth, rounded shape indicates water or abrasion, possibly with minor weathering or iron staining.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and lava flows. Individual pieces may range from modern beach material to fragments derived from rocks millions to billions of years old.
Uses & applications
Used as crushed aggregate, road stone, riprap, railway ballast, and building stone; attractive polished pieces are used decoratively and as inexpensive curios.
Geological facts
Basalt is Earth’s most abundant volcanic rock and commonly forms the ocean floor. A definitive identification would require tests such as streak, density, magnetism, and thin-section or chemical analysis.
Field identification & locations
Look for a uniformly dark, dense, fine-grained, non-layered stone; it may attract a weak magnet if iron-rich. Common locations include volcanic regions and many beaches or riverbeds worldwide.
