igneous
Porphyritic Basalt Pebble
Basalt with plagioclase phenocrysts
Hardness: 6 (Mohs scale), Color: Dark grey groundmass with white/light grey specks, Luster: Dull/earthy (weathered), Structure: Porphyritic (fine-grained matrix with larger visible crystals), Specific Gravity: 2.8 - 3.0
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey groundmass with white/light grey specks, Luster: Dull/earthy (weathered), Structure: Porphyritic (fine-grained matrix with larger visible crystals), Specific Gravity: 2.8 - 3.0
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 at depth before the final eruption. Common in volcanic terrains across all geological ages.
Uses & applications
Basalt is used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. Large boulders are used for riprap and landscaping. Porphyritic varieties are sometimes used for decorative garden stones.
Geological facts
This specimen is a 'water-worn' pebble, meaning its rounded shape is a result of mechanical weathering in a high-energy environment like a riverbed or a beach. The white spots are typically lath-shaped plagioclase feldspar crystals.
Field identification & locations
Identified by its dark, dense matrix and contrasting light-colored crystal inclusions. Commonly found on beaches near volcanic coastlines or in glacial till. Collectors value them as 'pocket stones' or for lapidary tumbling.
