igneous
Basalt Pebble
Extrusive mafic igneous rock
Hardness: 6 (Mohs), Color: Dark grey to black, Luster: Dull/Earthy, Crystal Structure: Aphanitic (fine-grained), Cleavage: None, Specific Gravity: 2.8-3.0
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Physical properties
Hardness: 6 (Mohs), Color: Dark grey to black, Luster: Dull/Earthy, 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 specimen appears river-worn or weathered into a rounded pebble shape.
Uses & applications
Used in construction as road base, concrete aggregate, and railroad ballast. Small polished pebbles are used in landscaping and aquariums.
Geological facts
Basalt is the most common volcanic rock on Earth and also makes up most of the ocean floor and much of the lunar surface (the dark patches or 'maria').
Field identification & locations
Identify by its dark color, heavy weight for its size, and lack of visible crystals. It is ubiquitous in volcanic regions and riverbeds downstream from volcanic mountains.
