igneous
Beach Pebble (Basalt / Greywacke Gravel)
Mafic Volcanic Rock (Basalt)
Hardness 5.5-6.0 Mohs, dark grey to charcoal color, dull to sub-vitreous earthy luster, fine-grained microcrystalline texture, rounded kidney or crescent shape smoothed by water erosion.
Identified More igneous →
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Physical properties
Hardness 5.5-6.0 Mohs, dark grey to charcoal color, dull to sub-vitreous earthy luster, fine-grained microcrystalline texture, rounded kidney or crescent shape smoothed by water erosion.
Formation & geological history
Formed from rapid cooling of mafic lava, subsequently broken down and smooth-rounded by wave action and stream transport along coasts or rivers over thousands to millions of years.
Uses & applications
Landscaping, aquarium decor, decorative rock collections, novelty items (due to unique curved/cashew shape).
Geological facts
Naturally rounded rocks with unique curved or hooked shapes are often formed when wave action erodes a pebble along original natural fracturing or fossil mold lines.
Field identification & locations
Look for smooth, water-worn texture and dark fine-grained matrix on beaches, riverbeds, and glacial till deposits.
