igneous
Basalt Pebble
Mafic Volcanic Rock (Basalt)
Hardness: 6.0 on Mohs scale; Color: Dark grey to black, occasionally weathered to brown; Luster: Dull or earthy when unpolished; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8–3.0
- Hardness
- 6
- Color
- Dark grey to black, occasionally weathered to brown
- Luster
- Dull or earthy when unpolished
Identified More igneous →
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Physical properties
Hardness: 6.0 on Mohs scale; Color: Dark grey to black, occasionally weathered to brown; Luster: Dull or earthy when unpolished; 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 smoothed and rounded by water erosion (abrasion) on a beach or riverbed.
Uses & applications
Used primarily in construction (crushed stone), road base, and as decorative landscaping stones. Highly polished basalt is sometimes used in jewelry or as hot stones in massage therapy.
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 (relative to size), and fine-grained texture. It often shows small holes called vesicles where gas bubbles were trapped during cooling. Found commonly on volcanic coastlines from Iceland to Hawaii.
