igneous
Dark volcanic rock, likely basalt
Basalt; primarily plagioclase feldspar and pyroxene, with possible olivine and alteration minerals
AI-generated identification · may be incorrectFine-grained, dark gray to brown, mostly dull to slightly vitreous; typically hardness 6–7 Mohs, dense, massive, and lacking visible cleavage. The pale curved vein or coating may be calcite, silica, or weathering mineralization.
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Physical properties
Fine-grained, dark gray to brown, mostly dull to slightly vitreous; typically hardness 6–7 Mohs, dense, massive, and lacking visible cleavage. The pale curved vein or coating may be calcite, silica, or weathering mineralization.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. Exact identification and age require a fresh surface, streak, density, and thin-section or chemical analysis.
Uses & applications
Used extensively as crushed aggregate, road stone, ballast, riprap, and occasionally dimension stone; specimens have modest collecting value.
Geological facts
Basalt commonly contains tiny crystals that are difficult to see without magnification and may weather to brown or reddish surfaces. Its dark color and high density distinguish it from many lighter volcanic rocks.
Field identification & locations
Look for a heavy, dark, fine-grained matrix with small feldspar crystals and no obvious sedimentary layering; magnetic response may occur if magnetite is present. Common worldwide in volcanic regions and oceanic crust.
