igneous
River Cobble / Basalt Pebble
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6-7 Mohs scale. Color: Dark gray to brownish-black with weathered surface. Luster: Dull, earthy to sub-metallic when wet. Texture: Fine-grained (aphanitic), rounded and smoothed by water erosion. Specific gravity: 2.8 - 3.0.
- Hardness
- 6-7 Mohs scale
- Color
- Dark gray to brownish-black with weathered surface
- Luster
- Dull, earthy to sub-metallic when wet
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Physical properties
Hardness: 6-7 Mohs scale. Color: Dark gray to brownish-black with weathered surface. Luster: Dull, earthy to sub-metallic when wet. Texture: Fine-grained (aphanitic), rounded and smoothed by water erosion. Specific gravity: 2.8 - 3.0.
Formation & geological history
Formed through the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The rounded cobble shape was subsequently formed through mechanical weathering and transport in riverbeds or coastal environments.
Uses & applications
Commonly used in landscaping, river rock garden beds, decorative outdoor accents, construction aggregate, and ballast.
Geological facts
Smooth river stones like this take hundreds to thousands of years of tumbling in flowing water to achieve their rounded, polished shape. Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor.
Field identification & locations
Identified in the field by its dark color, heavy feel relative to size, fine-grained texture, and rounded water-worn cobble morphology. Found abundantly along stream beds, rivers, glacial till deposits, and gravel bars.
