igneous
Vesicular basalt
Basalt, commonly plagioclase feldspar + pyroxene ± olivine
AI-generated identification · may be incorrectFine-grained, dark gray to brown, dull to subtly glassy, with numerous gas vesicles; hardness about 5–6 Mohs, massive texture, and no obvious cleavage. The rounded surface is naturally weathered or waterworn.
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Physical properties
Fine-grained, dark gray to brown, dull to subtly glassy, with numerous gas vesicles; hardness about 5–6 Mohs, massive texture, and no obvious cleavage. The rounded surface is naturally weathered or waterworn.
Formation & geological history
An extrusive mafic igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface, with trapped volcanic gases producing vesicles. Most examples formed in the Cenozoic or older volcanic episodes, but the image alone cannot establish age.
Uses & applications
Used as crushed aggregate, roadstone, riprap, and building stone; attractive polished pieces are collected but are generally not gemstone grade.
Geological facts
Vesicles may later fill with minerals such as zeolite, calcite, or quartz, forming amygdaloidal basalt. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its dark fine-grained matrix, low density relative to massive basalt, and rounded vesicles; it commonly occurs in volcanic fields, lava flows, river gravels, and beaches. A streak, magnetism, and a fresh broken surface help confirm it.
