igneous
Basalt Pebble
Basalt (Aphanitic Mafic Volcanic Rock)
Hardness: 6 on the Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained, invisible to the naked eye); Cleavage: None; Specific gravity: 2.8 - 3.0.
- Hardness
- 6 on the Mohs scale
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 on the Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained, invisible to the naked eye); 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. Basalt is the most common rock type on the Earth's crust, found in oceanic plates and volcanic regions.
Uses & applications
Commonly used in construction as aggregate for concrete and asphalt, as road base, and for railroad ballast. High-quality smooth pebbles are often sold for decorative landscaping, aquariums, and hot stone massage therapy.
Geological facts
Basalt also makes up the majority of the ocean floor and has been found on the Moon and Mars. When basalt cools slowly in thick layers, it can form striking hexagonal columns, such as those at the Giant's Causeway.
Field identification & locations
Identify in the field by its high density (heavy for its size), dark color, and fine-grained texture. It often feels slightly gritty despite a polished appearance. Commonly found along coastlines, riverbeds, and volcanic fields.
