igneous
Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 5-6 Mohs; Color: Black, dark gray, or dark green; Luster: Dull to sub-metallic; Crystal structure: Fine-grained/Aphanitic (crystals invisible to the naked eye); Specific gravity: 2.8 - 3.0
- Hardness
- 5-6 Mohs
- Color
- Black, dark gray, or dark green
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 Mohs; Color: Black, dark gray, or dark green; Luster: Dull to sub-metallic; Crystal structure: Fine-grained/Aphanitic (crystals invisible to the naked eye); 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. It is the most common volcanic rock on Earth, making up most of the ocean floor and oceanic crust.
Uses & applications
Extensively used in construction, road base, concrete aggregate, railroad ballast, and ground as a fertilizer. High-quality smooth stones are used for hot stone massage; some varieties are used for jewelry or outdoor landscaping.
Geological facts
Basalt is also found on other planets, including Venus and Mars, and covers large parts of the Moon (the dark plains called Maria). It is the most common rock type in the Earth's crust.
Field identification & locations
Identify by its dark color, heavy weight (density), and lack of visible crystals. It is commonly found at beaches, riverbeds, and volcanic regions. Small, water-worn pebbles like the one pictured are frequently found along coastlines.
