Glacial Cobble (likely Granitic or Gneissic)

Physical Properties

Hardness: 6-7 (Mohs scale), Color: Mottled grey, white, and dark green/black, Luster: Dull to sub-vitreous, Structure: Phaneritic (coarse-grained) to foliated, Cleavage: Poor, Specific Gravity: approx. 2.6-2.7

Formation & Geological History

Formed from slow-cooling magma (igneous) or extreme pressure/heat (metamorphic). This specific specimen was likely rounded and transported by glacial ice movement during the last ice age (Pleistocene).

Uses & Applications

Mainly used as decorative landscaping rocks, river rock features, or as aggregate in heavy construction.

Geological Facts

The rounded, 'pillowy' shape suggests this rock has been tumbled over great distances by water or moving ice, creating a smooth surface compared to freshly fractured stones.

Field Identification & Locations

Look for the speckled 'salt and pepper' appearance of quartz, feldspar, and mica. It is most commonly found in glacial till or riverbeds. Collectors can identify it by its high hardness and lack of reaction to acid.

Identified on: 5/18/2026

Mode: Standard