igneous
River Cobble (Basalt / Gabbro)
Mafic Igneous Rock (Basaltic / Gabbroic Composition)
Hardness: 6-7 Mohs scale. Color: Dark gray, charcoal, to black with subtle mottling. Luster: Dull to sub-vitreous, silky when wet. Texture: Fine to medium-grained, rounded and smoothed by water erosion. Cleavage: Non-cleaving, conchoidal to uneven fracture. Specific gravity: 2.8 - 3.0.
- Hardness
- 6-7 Mohs scale
- Color
- Dark gray, charcoal, to black with subtle mottling
- Luster
- Dull to sub-vitreous, silky when wet
Identified More igneous →
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Physical properties
Hardness: 6-7 Mohs scale. Color: Dark gray, charcoal, to black with subtle mottling. Luster: Dull to sub-vitreous, silky when wet. Texture: Fine to medium-grained, rounded and smoothed by water erosion. Cleavage: Non-cleaving, conchoidal to uneven fracture. Specific gravity: 2.8 - 3.0.
Formation & geological history
Formed through the rapid cooling of mafic lava at Earth's surface (basalt) or slow cooling of magma at depth (gabbro). The rounded shape is caused by natural stream or beach abrasion over thousands to millions of years.
Uses & applications
Used in landscaping, decorative stone displays, construction aggregate, aquarium decor, and tumbling projects. High heat retention makes smooth river cobbles popular for hot stone massage therapy.
Geological facts
Smooth river cobbles acquire their rounded shape through attrition, where rocks tumble over one another in flowing water. Dark mafic rocks like basalt contain high amounts of iron and magnesium, giving them their characteristic dark coloration.
Field identification & locations
Identified by its well-rounded, smooth pebble/cobble morphology, heavy weight relative to size, and dark gray-black color. Commonly found along stream beds, riverbanks, and coastal beaches worldwide.
