igneous
Vesicular basalt (likely weathered volcanic rock)
Basalt; chiefly plagioclase, clinopyroxene, and often olivine (approx. 45–52% SiO₂)
AI-generated identification · may be incorrectDark gray-brown, fine-grained and locally vesicular, with a dull to slightly vitreous luster; typical hardness 5–6 Mohs and specific gravity about 2.8–3.0. The visible rounded cavities and weathered surface are consistent with gas-rich lava, though the photo is not sufficient for a definitive identification.
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Physical properties
Dark gray-brown, fine-grained and locally vesicular, with a dull to slightly vitreous luster; typical hardness 5–6 Mohs and specific gravity about 2.8–3.0. The visible rounded cavities and weathered surface are consistent with gas-rich lava, though the photo is not sufficient for a definitive identification.
Formation & geological history
Forms when mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows, cinder fields, and oceanic crust; specimens may range from modern eruptions to Precambrian in age.
Uses & applications
Used extensively as crushed aggregate, road base, concrete material, riprap, and building stone; attractive polished varieties are used decoratively. This irregular weathered specimen has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and forms much of the ocean floor. Vesicles may later become filled with minerals such as zeolite, calcite, or quartz.
Field identification & locations
Look for a dark, dense fine-grained matrix, tiny gas cavities, and absence of obvious layered sedimentary texture; a streak, fresh broken surface, and magnet test can help. Common worldwide, especially volcanic regions and oceanic islands.
