igneous
Basalt River Pebble
Basalt (Mafic Volcanic Rock)
Hardness: 5-6 (Mohs), Color: Dark grey to black, Luster: Dull/Sub-metallic, Crystal Structure: Aphanitic (fine-grained), Cleavage: None, Specific Gravity: 2.8-3.0
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Physical properties
Hardness: 5-6 (Mohs), Color: Dark grey to black, Luster: Dull/Sub-metallic, Crystal Structure: Aphanitic (fine-grained), Cleavage: None, 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. This specific specimen has been rounded by fluvial (river) erosion over a period of years to centuries.
Uses & applications
Commonly used as crushed stone for road base, concrete aggregate, and railroad ballast; small pebbles like this are used in landscaping or as decorative stones.
Geological facts
Basalt is the most common rock in Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon and Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight for its size, and lack of visible crystals. It is found globally in volcanic regions and riverbeds downstream from volcanic mountains.
