igneous
Basalt Pebble / Black River Stone
Aphanitic Mafic Igneous Rock (Basalt)
Hardness: 6-7 on Mohs scale. Color: Dark grey to black. Luster: Sub-metallic to dull, satin-like when river-worn. Texture: Fine-grained (aphanitic), highly rounded and smoothed by water erosion. Density: High (2.8-3.0 g/cm³).
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black
- Luster
- Sub-metallic to dull, satin-like when river-worn
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark grey to black. Luster: Sub-metallic to dull, satin-like when river-worn. Texture: Fine-grained (aphanitic), highly rounded and smoothed by water erosion. Density: High (2.8-3.0 g/cm³).
Formation & geological history
Formed from rapid cooling of mafic lava rich in magnesium and iron at or near Earth's surface. Water action in streams, rivers, or beach coastlines subsequently rounded and polished the rock into a smooth pebble.
Uses & applications
Commonly used in landscaping, river rock decor, aquariums, masonry, and massage stones due to high heat retention. Highly rounded dark pebbles are also collected for rock tumbling and craft projects.
Geological facts
Smooth river pebbles like this acquire their shape through attrition—collision with other rocks and sand in moving water over hundreds to thousands of years. Basalt is the most common rock type in the Earth's crust, forming most of the ocean floors.
Field identification & locations
Identified by its dark black/grey color, dense feeling, lack of visible coarse crystals, and exceptionally smooth rounded shape. Found globally along riverbeds, lakeshores, gravel bars, and alluvial deposits.
