igneous
Basalt River Rock
Basalt (Aphanitic Mafic Igneous Rock)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black; Luster: Dull/Earthy; Crystal structure: Fine-grained/Aphanitic (minerals too small to see with the naked eye); Cleavage: None; Specific Gravity: 2.8–3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black; Luster: Dull/Earthy; Crystal structure: Fine-grained/Aphanitic (minerals too small to see with 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. This specific specimen has been chemically and mechanically weathered by water (fluvial processes), resulting in its smooth, rounded 'river rock' shape.
Uses & applications
Used extensively in construction as crushed stone (aggregate) for roads and concrete. Smooth, rounded specimens are popular in landscaping, xeriscaping, and massage therapy (hot stone massage).
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. Most basaltic lava on Earth is less than 200 million years old.
Field identification & locations
Identify by its dark color, heavy weight (high density), and fine-grained texture. It will not scratch easily with a steel blade. Found globally in volcanic regions, riverbeds, and as common landscaping gravel.
