igneous
Vesicular basalt (possibly weathered amygdaloidal basalt)
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine, with weathering minerals filling vesicles
AI-generated identification · may be incorrectDark gray, fine-grained, dull to slightly vitreous; typically hardness 5–6, massive with small gas cavities (vesicles) and irregular fracture. The elongated cavities appear partly filled by rusty iron oxides or secondary minerals.
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Physical properties
Dark gray, fine-grained, dull to slightly vitreous; typically hardness 5–6, massive with small gas cavities (vesicles) and irregular fracture. The elongated cavities appear partly filled by rusty iron oxides or secondary minerals.
Formation & geological history
An extrusive igneous rock formed when low-viscosity mafic lava cooled rapidly at or near Earth’s surface. Basalt ranges from Archean to modern age and commonly forms volcanic flows, oceanic crust, and cinder cones.
Uses & applications
Widely used as crushed aggregate, road metal, riprap, and dimension stone; attractive polished pieces are collected, though this specimen has modest gem value.
Geological facts
Vesicles record gas escaping from lava; rusty cavity linings commonly result from oxidation of iron-bearing minerals. If cavities are mineral-filled, the rock is specifically called amygdaloidal basalt.
Field identification & locations
Common in volcanic regions and oceanic crust, including Hawaii, Iceland, the Deccan Traps, and the Columbia River Basalt Group. Identify it by its dark fine-grained groundmass, heavy weight, lack of visible quartz, and vesicles; a streak and hardness test can help distinguish it from slag.
