igneous
Granite Cobble / Gneiss (River Rock)
Granitic Gneiss / Felsic Igneous Rock
Hardness 6-7 on Mohs scale; medium-to-coarse grained texture; earthy to dull luster when wet; composed mainly of quartz, potassium feldspar, plagioclase, and minor biotite or hornblende; colors range from tan, brownish-gold, to dark grey with iron oxide staining.
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Physical properties
Hardness 6-7 on Mohs scale; medium-to-coarse grained texture; earthy to dull luster when wet; composed mainly of quartz, potassium feldspar, plagioclase, and minor biotite or hornblende; colors range from tan, brownish-gold, to dark grey with iron oxide staining.
Formation & geological history
Formed through slow crystallization of felsic magma deep beneath Earth's surface (or metamorphism of granitic rocks), subsequently exposed to erosion and water-rounding in a riverbed or stream environment.
Uses & applications
Landscaping, river rock decoration, masonry, aggregate material for construction, and decorative garden features.
Geological facts
Stream-smoothed rocks like this show a natural contrast between smooth, water-worn outer rinds and rougher broken interior surfaces that reveal their crystalline mineral grains.
Field identification & locations
Identified by visible interlocking mineral grains (quartz, feldspar, biotite) and smooth water-worn surface contour. Commonly found in alluvial river deposits, glacial till, and stream beds near mountainous areas.
