igneous
Basalt with Plagioclase Phenocrysts (Porphyritic Basalt)
Porphyritic Basalt
AI-generated identification · may be incorrectHardness 6-7 (Mohs); Color: Dark grey to brown groundmass with white/pale grey phenocrysts; Luster: Dull/earthy; Texture: Porphyritic (fine-grained matrix with larger visible crystals); Composition: Plagioclase, pyroxene, and olivine.
- Luster
- Dull/earthy
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Physical properties
Hardness 6-7 (Mohs); Color: Dark grey to brown groundmass with white/pale grey phenocrysts; Luster: Dull/earthy; Texture: Porphyritic (fine-grained matrix with larger visible crystals); Composition: Plagioclase, pyroxene, and olivine.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The larger white crystals (phenocrysts) formed slowly underground before the final eruption. Most common in volcanic regions and ocean floors.
Uses & applications
Used primarily as crushed stone for construction, road base, and concrete aggregate. Smooth river-worn specimens are popular for landscape decoration and as tactile 'worry stones' for collectors.
Geological facts
Basalt is the most common rock type in the Earth's crust and also makes up most of the surface of the Moon and Mars. Specimens like this have been rounded and smoothed by water transport in a river or beach environment.
Field identification & locations
Look for a dark, dense matrix containing small, light-colored rectangular or lath-shaped crystals. Found globally in volcanic provinces, especially in the Pacific Northwest, Hawaii, and along riverbanks near ancient lava flows.
