igneous
Vesicular basalt (likely weathered scoriaceous basalt)
Basalt; predominantly plagioclase, clinopyroxene, and volcanic glass, with possible iron oxides and secondary zeolite minerals
AI-generated identification · may be incorrectDark gray to purplish-gray, fine-grained, vesicular, and dull to weakly subvitreous; typically hardness 5–6, no obvious cleavage, and specific gravity about 2.7–3.0. The cavities are irregular gas bubbles, some partly filled or weathered.
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Physical properties
Dark gray to purplish-gray, fine-grained, vesicular, and dull to weakly subvitreous; typically hardness 5–6, no obvious cleavage, and specific gravity about 2.7–3.0. The cavities are irregular gas bubbles, some partly filled or weathered.
Formation & geological history
An extrusive mafic igneous rock formed when low-viscosity basaltic lava cooled rapidly at or near the surface, trapping volcanic gas. It may range from recent volcanic material to ancient basalt, but age cannot be determined from the photograph.
Uses & applications
Commonly used as crushed aggregate, road base, riprap, railroad ballast, and landscaping stone; attractive vesicular pieces may be collected, though this specimen has limited gem value.
Geological facts
Its purple coloration likely reflects oxidation or weathering of iron-bearing minerals. Vesicles distinguish it from most dense basalt and from pumice, which is generally much lighter and more silica-rich.
Field identification & locations
Identify it by its dark fine-grained groundmass, heavy feel, and abundant rounded cavities; a streak, magnetism test, and fresh fracture would help confirm it. Common near volcanic fields, lava flows, and cinder cones.
