igneous
River Pebble (Basalt or Gabbro)
Mafic Igneous Rock
Hardness: 5-7 Mohs; Color: Dark grey to black with greenish mottling; Luster: Sub-vitreous to dull (appears glossy when wet); Texture: Fine to medium-grained; SG: 2.8-3.0.
- Hardness
- 5-7 Mohs
- Color
- Dark grey to black with greenish mottling
- Luster
- Sub-vitreous to dull (appears glossy when wet)
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Physical properties
Hardness: 5-7 Mohs; Color: Dark grey to black with greenish mottling; Luster: Sub-vitreous to dull (appears glossy when wet); Texture: Fine to medium-grained; SG: 2.8-3.0.
Formation & geological history
Formed from the cooling of magnesium-rich and iron-rich magma or lava. These rounded shapes are created through fluvial erosion, where the rock is tumbled in a riverbed over years.
Uses & applications
Commonly used in landscaping, aquarium decor, and as decorative aggregate in construction (terrazzo or exposed concrete). Small, smooth specimens are popular for rock tumbling.
Geological facts
The rounded shape is a result of a process called attrition, where rocks collide with each other in moving water, wearing down sharp edges into a smooth 'river stone' finish.
Field identification & locations
Look for a dark, heavy, dense appearance. It will be much harder than a fingernail and will not fizz with acid. Very common in riverbeds and glacial deposits.
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