igneous
Basalt (River Pebble)
Basalt (extrusive mafic rock)
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/earthy; Crystal Structure: Fine-grained/Aphanitic (crystals too small to see with naked eye); Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull/earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/earthy; Crystal Structure: Fine-grained/Aphanitic (crystals too small to see with naked eye); Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. This specific specimen has been rounded and smoothed by water erosion through river or glacial transport over thousands of years.
Uses & applications
Commonly used as aggregate in road construction, concrete, and railroad ballast. Smooth river-worn basalt is often used in landscaping, decorative aquariums, and hot-stone massage therapy.
Geological facts
Basalt is the most common rock type on the Earth's surface and covers most of the ocean floor. It is also found on the Moon and Mars, where the lunar 'maria' (dark patches) are composed of ancient basaltic lava flows.
Field identification & locations
Identify in the field by its uniform dark color, lack of visible crystals, and significant weight compared to lighter rocks like sandstone. Look for smooth, rounded shapes near riverbeds or coastal areas. Collectors value it for tumbling or as filler in decorative displays.
