igneous
Rounded dark basalt pebble
Basalt, a fine-grained mafic volcanic rock
AI-generated identification · may be incorrectTypically 2–3 inches? The specimen appears small and water-worn, dark gray to black with a dull-to-subvitreous luster and very fine interlocking crystals. Hardness is about 6 on Mohs; composition is mainly plagioclase feldspar, pyroxene, and commonly olivine, with no visible cleavage.
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Physical properties
Typically 2–3 inches? The specimen appears small and water-worn, dark gray to black with a dull-to-subvitreous luster and very fine interlocking crystals. Hardness is about 6 on Mohs; composition is mainly plagioclase feldspar, pyroxene, and commonly olivine, with no visible cleavage.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined from the image; the rounded shape indicates transport by water or abrasion.
Uses & applications
Used extensively as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive polished pieces are collected but are generally not jewelry gemstones.
Geological facts
Its dark color and fine grain reflect rapid cooling and high mafic-mineral content. Vesicles may occur, but none are clearly visible here.
Field identification & locations
Identify it by its heavy feel, dark fine-grained texture, lack of obvious layers or large crystals, and resistance to scratching; a steel file may barely scratch it. Common worldwide, especially near volcanic regions, river gravels, and beaches.
