igneous
Glacial Cobble (likely Granitic or Gneissic)
Granite/Gneiss Glacial Erratic
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
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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.
