River Rock (Basalt/Greywacke)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Dark grey to charcoal; Luster: Dull/Earthy; Crystal structure: Fine-grained (Aphanitic); Specific Gravity: 2.7-3.0.

Formation & Geological History

Formed through the rapid cooling of basaltic lava or as a dense sedimentary deposit, later rounded by mechanical weathering in high-energy water environments like rivers or beaches.

Uses & Applications

Commonly used in landscaping, decorative stone mulch, construction aggregate, and as a natural massage or spa stone.

Geological Facts

River rocks are geological 'history books' that can travel hundreds of miles from their source, becoming increasingly smooth and elliptical the further they move from their outcrop.

Field Identification & Locations

Identify in the field by its smooth, water-worn texture and lack of visible grains to the naked eye. Found worldwide in riverbeds, glacial tills, and coastal areas.

Identified on: 4/26/2026

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