Granite Cobble / Rounded Granite River Rock

Physical Properties

Hardness: 6-7 (Mohs scale), Color: tan, beige, pinkish with black/dark specks, Luster: dull to slightly shiny on distinct mineral grains, Crystal structure: coarse-grained (phaneritic), Specific gravity: 2.6-2.7. Shape is highly rounded.

Formation & Geological History

Formed from the slow cooling of magma deep underground, creating a coarse-grained igneous rock. The perfectly round shape suggests significant transportation and weathering in a high-energy water environment, like a fast-moving river or glacial meltwater.

Uses & Applications

Commonly used in landscaping, construction aggregate, gabion walls, or decorative garden elements. Large perfectly round specimens can be collector curiosities.

Geological Facts

The high degree of rounding (sphericity) indicates it spent a long time being tumbled against other rocks in a riverbed or glacial outwash before being deposited.

Field Identification & Locations

Identify by its coarse-grained texture (visible interlocking crystals of feldspar, quartz, and mica) and its extreme roundness. Commonly found in riverbeds, glacial moraines, and outwash plains.

Identified on: 4/26/2026

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