igneous
Basalt (with Porphyritic or Amygdaloidal features)
Mafic Volcanic Rock (Basalt)
Hardness: 5-6 (Mohs); Color: Dark gray, black, or blue-gray; Luster: Dull to earthy (weathered); Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0
- Hardness
- 5-6 (Mohs)
- Color
- Dark gray, black, or blue-gray
- Luster
- Dull to earthy (weathered)
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark gray, black, or blue-gray; Luster: Dull to earthy (weathered); Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the earth's surface. Often found as water-worn pebbles in glacial or beach deposits.
Uses & applications
Commonly used in construction as aggregate, road base, and concrete; occasionally used in decorative landscaping or as worry stones when river-worn.
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 compared to other pebbles, and fine-grained texture. Commonly found on Great Lakes beaches and volcanic coastal regions.
