igneous
Basalt (Water-worn pebble)
Basalt (Mafic Extrusive Rock)
Hardness: 6-7 Mohs; Color: Dark grey to black, often weathering to green or brown; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8 - 3.0.
- Hardness
- 6-7 Mohs
- Color
- Dark grey to black, often weathering to green or brown
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to black, often weathering to green or brown; Luster: Dull to sub-vitreous; 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 or near the Earth's surface. This specimen appears to have been rounded and smoothed by water erosion in a river, stream, or coastal environment throughout the Holocene epoch.
Uses & applications
Used extensively in construction as aggregate for concrete and asphalt, as road base, and for railroad ballast. High-quality basalt fibers are used in aerospace and automotive industries.
Geological facts
Basalt is the most common rock type in Earth's crust and makes up most of the ocean floor. It is also found on the Moon and Mars, forming the dark plains known as 'maria'.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It will not react with acid (unlike limestone) and is difficult to scratch with a steel knife. Common in volcanic regions such as the Pacific Northwest, Iceland, and Hawaii.
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