igneous
Beach Pebble (Basalt/Porphyry)
Weathered Basalt or Porphyry
Hardness around 6-7 on Mohs scale, dark reddish-brown to dark gray color with fine-grained texture and small lighter mineral inclusions (phenocrysts). Dull to waxy luster due to water-smoothing, sub-angular to rounded shape.
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Physical properties
Hardness around 6-7 on Mohs scale, dark reddish-brown to dark gray color with fine-grained texture and small lighter mineral inclusions (phenocrysts). Dull to waxy luster due to water-smoothing, sub-angular to rounded shape.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. Subsequent erosion by wave action or rivers rounded and smoothed the rock into a pebble over thousands to millions of years.
Uses & applications
Commonly used in landscaping, river rock aggregate for construction, aquariums, lapidary polishing practice, and casual rock collecting.
Geological facts
Beach pebbles acquire their smooth, rounded shapes through mechanical weathering known as attrition, where stones collide with each other in coastal surf or river currents.
Field identification & locations
Identify in the field by its dark fine-grained matrix, smooth water-worn surface, and small embedded mineral phenocrysts. Found in abundance along coastal shores, gravel bars, and river beds globally.
