igneous
Porphyritic basalt (likely amygdaloidal basalt)
Basaltic rock dominated by plagioclase feldspar and pyroxene, with possible vesicle-filling zeolite/calcite
AI-generated identification · may be incorrectDark reddish-brown, fine-grained volcanic matrix containing abundant rounded to angular lighter-brown phenocrysts or amygdules; typically Mohs 5–6, dull to subtly vitreous luster, little visible cleavage, and specific gravity about 2.8–3.1.
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Physical properties
Dark reddish-brown, fine-grained volcanic matrix containing abundant rounded to angular lighter-brown phenocrysts or amygdules; typically Mohs 5–6, dull to subtly vitreous luster, little visible cleavage, and specific gravity about 2.8–3.1.
Formation & geological history
Formed when basaltic lava cooled at or near the surface, preserving larger early-formed crystals or gas cavities later filled by secondary minerals. It is generally Cenozoic or younger, though similar textures occur throughout the geologic record.
Uses & applications
Used as decorative stone, lapidary material, aggregate, and collector specimens; quality pieces may be polished for display.
Geological facts
The crowded rounded features may represent feldspar phenocrysts, altered vesicles, or both; exact identification requires a fresh break, hardness tests, and thin-section or chemical analysis.
Field identification & locations
Common in ancient lava flows and volcanic provinces worldwide, including India, the western United States, Mexico, and Iceland. Look for a dense volcanic groundmass, magnetic response, and crystals/amygdules that contrast with it.
