igneous
Basalt (Black Beach Pebble)
Basalt
Hardness: 5-6 (Mohs scale), Color: Black or dark gray, Luster: Dull to sub-glassy when wet, Crystal structure: Aphanitic (fine-grained), Cleavage: None (conchoidal fracture), Specific Gravity: 2.8-3.0
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Physical properties
Hardness: 5-6 (Mohs scale), Color: Black or dark gray, Luster: Dull to sub-glassy when wet, Crystal structure: Aphanitic (fine-grained), Cleavage: None (conchoidal fracture), Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. High-energy beach environments often tumble these into smooth, rounded pebbles.
Uses & applications
Commonly used as construction aggregate, road base, and decorative landscaping stones. Highly polished versions are used in massage therapy (hot stones) and craft jewelry.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon and Mars.
Field identification & locations
Identify by its dark color, heavy weight for its size, and absence of visible mineral crystals to the naked eye. Commonly found on volcanic coastlines.
