igneous
Basalt (River Stone)
Basalt (Aphanitic mafic volcanic rock)
Hardness: 6.0 on Mohs scale; Color: Dark grey to black; Luster: Dull/Sub-metallic; Crystal Structure: Fine-grained (Aphanitic); Cleavage: None; Specific Gravity: 2.8-3.0.
- Hardness
- 6
- Color
- Dark grey to black
- Luster
- Dull/Sub-metallic
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Physical properties
Hardness: 6.0 on Mohs scale; Color: Dark grey to black; Luster: Dull/Sub-metallic; Crystal Structure: Fine-grained (Aphanitic); 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 further shaped by fluvial (river) erosion, rounding its edges over thousands of years.
Uses & applications
Used extensively in construction as road base, concrete aggregate, and railroad ballast. Smooth river-worn basalt is highly valued for decorative landscaping, 'hot stone' massage therapy, and aquarium decor.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor. It is also found on the Moon and Mars, forming the dark 'Maria' plains visible from Earth.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and lack of visible large crystals. Commonly found in volcanic regions and riverbeds. Look for a fine-grained, salt-and-pepper texture when viewed under a lens.
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