igneous
Weathered mafic rock, likely basalt
Basalt; fine-grained plagioclase–pyroxene rock, commonly with olivine and iron oxides
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to slightly waxy luster, rounded by abrasion and weathering; typically Mohs hardness 5–6, no visible cleavage, and specific gravity about 2.8–3.0. The surface appears oxidized and may contain small vesicles or alteration patches.
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Physical properties
Dark gray-brown, fine-grained, dull to slightly waxy luster, rounded by abrasion and weathering; typically Mohs hardness 5–6, no visible cleavage, and specific gravity about 2.8–3.0. The surface appears oxidized and may contain small vesicles or alteration patches.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. The specimen is likely geologically ancient relative to its current weathering, but its exact age cannot be determined visually.
Uses & applications
Used as crushed aggregate, road material, riprap, railroad ballast, and dimension stone; attractive polished pieces may be sold as inexpensive decorative specimens. This small weathered pebble has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and iron-rich glass or oxides.
Field identification & locations
Identify it by its dark color, dense fine texture, lack of obvious large crystals, and heavy feel; a streak, magnetism test, and fresh broken surface would improve confidence. Common in volcanic regions and glacial or stream gravels worldwide.
