igneous
Porphyritic basalt with quartz or feldspar phenocrysts
Basaltic rock; likely vesicular/porphyritic basalt, with pale mineral inclusions possibly quartz, feldspar, or calcite
AI-generated identification · may be incorrectDark gray-black, fine-grained groundmass with rounded white-to-pink inclusions; typically ~5–6 Mohs, dull to subtly vitreous luster, and no obvious cleavage. The inclusions appear partly weathered or polished, so exact mineral identification from the image is uncertain.
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Physical properties
Dark gray-black, fine-grained groundmass with rounded white-to-pink inclusions; typically ~5–6 Mohs, dull to subtly vitreous luster, and no obvious cleavage. The inclusions appear partly weathered or polished, so exact mineral identification from the image is uncertain.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, trapping larger crystals or later-filling cavities; commonly Cenozoic to Precambrian in age depending on locality. It may also be a river-worn volcanic pebble.
Uses & applications
Used as aggregate, road stone, riprap, and occasionally as a decorative or lapidary stone; individual pebbles have modest collecting value.
Geological facts
The contrasting pale spots are characteristic of porphyritic texture or amygdules—gas bubbles later filled by minerals. A streak, hardness test, and close examination of crystal shape would distinguish quartz, feldspar, and calcite.
Field identification & locations
Look for a dense dark volcanic matrix, small crystals, and rounded weathering; basalt is widespread in volcanic terrains and stream gravels worldwide. Calcite inclusions fizz in dilute acid, while quartz scratches glass and does not fizz.
