igneous
Basalt
Basalt (Mafic Extrusive)
Hardness (Mohs): 6; Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8 - 3.1
- Color
- Dark gray to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness (Mohs): 6; Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8 - 3.1
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Common in oceanic crust and mid-ocean ridges.
Uses & applications
Extensively used in construction for road base, concrete aggregate, and railroad ballast. Also used as dimension stone for tiles and monuments.
Geological facts
Basalt is the most common volcanic rock on Earth. It also covers large areas of the Moon (the Lunar Maria) and Mars.
Field identification & locations
Identify by its dark color, heavy weight (relative to size), and fine-grained texture. It often shows smooth, water-worn surfaces if found in riverbeds like the specimen pictured.
