igneous
Basalt cobble in glacial till or gravel
Basalt, a fine-grained mafic volcanic rock; likely an altered basalt clast
AI-generated identification · may be incorrectTypically Mohs hardness 5–6, dark gray to black, with dull to subtly vitreous luster and microscopic interlocking plagioclase–pyroxene crystals; the pale tan surface may be weathering rind or adhered sediment. No obvious cleavage is visible.
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Physical properties
Typically Mohs hardness 5–6, dark gray to black, with dull to subtly vitreous luster and microscopic interlocking plagioclase–pyroxene crystals; the pale tan surface may be weathering rind or adhered sediment. No obvious cleavage is visible.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic provinces and oceanic crust. This cobble was likely transported and rounded by glaciers, rivers, or coastal processes; its precise age cannot be determined from the image.
Uses & applications
Crushed basalt is used as road aggregate, railroad ballast, concrete aggregate, and construction stone; attractive pieces may be collected, but this specimen has little gemstone value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly contains olivine, pyroxene, and calcium-rich plagioclase. Weathering can produce tan, brown, or reddish coatings rich in iron oxides.
Field identification & locations
Identify it by its dense, dark color, fine texture, and lack of large crystals; a streak test is usually gray, and it commonly attracts weakly to moderately to a magnet if iron-rich. Common sources include volcanic terrains and glacial deposits worldwide.
