metamorphic

River Cobble (likely Metasedimentary or Basalt)

Metamorphosed Siltstone or Altered Fine-grained Igneous Rock

AI-generated identification · may be incorrect

Hardness: 6-7 (scratches glass), Color: dark grey to greenish-black with orange/yellow veining, Luster: dull to waxy when wet, Structure: microcrystalline to massive with a quartz/feldspar vein.

Identified July 6, 2026More metamorphic →

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Physical properties

Hardness: 6-7 (scratches glass), Color: dark grey to greenish-black with orange/yellow veining, Luster: dull to waxy when wet, Structure: microcrystalline to massive with a quartz/feldspar vein.

Formation & geological history

Formed through original deposition or volcanic activity, followed by low-grade metamorphism and hydrothermal veining. Rounded by river or glacial transport over thousands of years.

Uses & applications

Landscape stone, lapidary practice (when polished), or geological educational sample.

Geological facts

The distinct yellow-orange line is a narrow hydrothermal vein mineralized with quartz or feldspar that filled a fracture in the parent rock prior to it being eroded and rounded into a cobble.

Field identification & locations

Identify by its rounded 'river rock' shape and the presence of cross-cutting veins. Found in riverbeds, glacial tills, and beaches globally. Durable and smooth surface.