igneous
Rounded river cobble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; exact identification requires testing
AI-generated identification · may be incorrectDark gray-green, very fine-grained, dull to weakly waxy luster, massive with no visible crystals or cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.7–3.0. Rust-brown patches likely iron-oxide weathering.
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Physical properties
Dark gray-green, very fine-grained, dull to weakly waxy luster, massive with no visible crystals or cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.7–3.0. Rust-brown patches likely iron-oxide weathering.
Formation & geological history
An ancient lava flow cooled rapidly at or near Earth’s surface, then weathered and was rounded by stream or coastal abrasion; age cannot be determined from the photograph.
Uses & applications
Commonly used as aggregate, riprap, landscaping stone, and decorative cobble; generally little jewelry or collector value unless unusually patterned or locality-specific.
Geological facts
The smooth shape records prolonged transport or wave action. Basalt is typically darker and denser, while andesite may be slightly lighter and more silica-rich.
Field identification & locations
Look for a uniformly fine texture, high density, absence of layering, and weak attraction to a magnet if magnetite is present; a streak, hardness, acid, and thin-section test would refine identification. Common worldwide in volcanic terrains and river gravels.
