igneous
River Stone (Grey Andesite)
Intermediate extrusive volcanic rock (Andesite)
Hardness: 6-7 (Mohs scale); Color: Grey with darker micro-phenocrysts; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.5-2.8
- Hardness
- 6-7 (Mohs scale)
- Color
- Grey with darker micro-phenocrysts
- Luster
- Dull/Earthy
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Grey with darker micro-phenocrysts; Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.5-2.8
Formation & geological history
Formed through the cooling of intermediate lava at volcanic arcs and subduction zones. This specific specimen has been Rounded and smoothed by fluvial (river) or coastal erosion processes.
Uses & applications
Commonly used in landscaping, as aquarium decor, or as aggregate in construction; polished versions are used for river rock massage or decorative pebble mosaics.
Geological facts
The name 'Andesite' is derived from the Andes Mountains in South America. Most river stones of this color are hundreds of thousands to millions of years old, having traveled miles from their volcanic source.
Field identification & locations
Identify in the field by its uniform grey color, fine texture, and smooth, water-worn surface. Often found in riverbeds, glacial tills, and along rocky shorelines.
