igneous
Rounded basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, commonly with olivine and magnetite
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, dull to subtly satin luster, hardness about 6 Mohs, no visible cleavage, and specific gravity around 2.8–3.0. The specimen is water-worn and may have a slightly metallic sparkle from magnetite or weathered surfaces.
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Physical properties
Typically dark gray to black, fine-grained, dull to subtly satin luster, hardness about 6 Mohs, no visible cleavage, and specific gravity around 2.8–3.0. The specimen is water-worn and may have a slightly metallic sparkle from magnetite or weathered surfaces.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. Its exact geological age cannot be determined from the photograph; the pebble was later rounded by stream, beach, or glacial transport.
Uses & applications
Crushed basalt is widely used for road aggregate, railway ballast, concrete, and construction stone; attractive pieces are sold as landscape or educational specimens rather than jewelry.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color and fine texture reflect rapid cooling and iron- and magnesium-rich magma.
Field identification & locations
Identify it by its dark, dense, fine-grained appearance, rounded abrasion, and lack of obvious large crystals; a streak test is usually gray, and it is not strongly magnetic unless magnetite-rich. Common in volcanic regions and gravel beaches worldwide.
