igneous
Tumbled basalt pebble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, and dull to subtly glassy; Mohs hardness about 5–6, no visible cleavage, and specific gravity around 2.8–3.0. The rounded shape and smooth surface indicate natural abrasion or tumbling, while the lighter circular area may be a weathered or polished surface.
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Physical properties
Typically dark gray to black, fine-grained, and dull to subtly glassy; Mohs hardness about 5–6, no visible cleavage, and specific gravity around 2.8–3.0. The rounded shape and smooth surface indicate natural abrasion or tumbling, while the lighter circular area may be a weathered or polished surface.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and continental lava flows. It ranges from modern to Precambrian in age; the specimen’s age cannot be determined visually.
Uses & applications
Used extensively as crushed aggregate, roadstone, concrete and asphalt material, ballast, and decorative landscaping stone. Small polished pieces are inexpensive collectibles or craft materials.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly forms extensive lava plateaus and the seafloor.
Field identification & locations
Identify it by its uniformly fine texture, dark color, high density, and lack of obvious grains or layering; a steel file should scratch it only slightly. Common worldwide, especially near volcanic regions and beaches or riverbeds.
