igneous
River Stone (Basalt or Siltstone)
Basalt (extrusive igneous rock) or fine-grained Siltstone
Hardness: 6-7 (Mohs), Color: Dark grey to charcoal black when wet, Luster: Dull (natural) to Vitreous (when wet), Crystal structure: Aphanitic (fine-grained), Cleavage: None, Specific Gravity: 2.8-3.0
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Physical properties
Hardness: 6-7 (Mohs), Color: Dark grey to charcoal black when wet, Luster: Dull (natural) to Vitreous (when wet), Crystal structure: Aphanitic (fine-grained), Cleavage: None, Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earths surface. This specimen has been smoothed and rounded by long-term hydraulic action in a river or coastal environment.
Uses & applications
Landscaping, decorative garden mulch, aquarium substrate, hot stone massage therapy, and occasionally used in construction aggregate.
Geological facts
Basalt is the most common rock type in the Earths crust and makes up most of the ocean floor. When these rocks are smoothed by rivers, they are often called river jacks or Mexican Beach Pebbles.
Field identification & locations
Identify by its smooth, ellipsoidal shape, dense weight, and uniform dark color. Commonly found in riverbeds, glacial tills, and along coastlines.
