Igneous
Basalt (Polished River Stone)
Mafic Volcanic Rock (primarily Plagioclase and Pyroxene)
Hardness: 5-6 (Mohs); Color: Dark grey to black; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None; Specific Gravity: 2.8–3.0
- Hardness
- 5-6 (Mohs)
- Color
- Dark grey to black
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey to black; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None; Specific Gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. This specimen shows signs of tumbling/polishing, likely from high-energy water environments like rivers or beaches.
Uses & applications
Commonly used in construction (aggregate), as a decorative landscaping stone, in hot stone massage therapies, and for 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, Mars, and Venus.
Field identification & locations
Identify in the field by its dark color, heavy weight for its size, and lack of visible crystals without a lens. Often found in volcanic regions or as rounded cobbles in riverbeds.
