igneous
Rounded granite or granitic gneiss cobble
Granite; mainly quartz, alkali feldspar, plagioclase, and mica
AI-generated identification · may be incorrectCoarse, speckled gray-tan granular texture with dull to weakly vitreous luster; likely Mohs 6–7, massive with no obvious cleavage, and specific gravity about 2.6–2.7. Its rounded shape and surface smoothing indicate stream or wave abrasion.
Identified More igneous →
Explore Rounded granite or granitic gneiss cobble in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Coarse, speckled gray-tan granular texture with dull to weakly vitreous luster; likely Mohs 6–7, massive with no obvious cleavage, and specific gravity about 2.6–2.7. Its rounded shape and surface smoothing indicate stream or wave abrasion.
Formation & geological history
Formed by slow crystallization of silica-rich magma deep in the crust, commonly during Precambrian to Phanerozoic continental-mountain-building events; later weathered and transported as a cobble.
Uses & applications
Used widely as crushed aggregate, building stone, paving, landscaping rock, and decorative cobbles; this ordinary specimen has little jewelry value.
Geological facts
Visible grains suggest an intrusive crystalline rock rather than sandstone; iron staining accounts for the warm brown coloration. A definitive distinction from granitic gneiss requires checking for foliation or banding on fresh surfaces.
Field identification & locations
Common in glacial deposits, riverbeds, beaches, and areas underlain by granite or gneiss, including many parts of North America and Europe. Look for interlocking quartz and feldspar grains; a streak, acid test, and fresh break can improve identification.
