igneous
Rounded granite or granitic gneiss cobble
Granite/granitic gneiss, chiefly quartz, feldspar, and mica
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray, fine-to-medium granular texture, dull to slightly sparkling luster, and no obvious cleavage as a whole. Rounded, smooth edges indicate water or glacial abrasion; individual feldspar grains may show cleavage.
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Physical properties
Hardness about 6–7 Mohs; gray, fine-to-medium granular texture, dull to slightly sparkling luster, and no obvious cleavage as a whole. Rounded, smooth edges indicate water or glacial abrasion; individual feldspar grains may show cleavage.
Formation & geological history
Formed when silica-rich magma slowly crystallized deep underground, likely hundreds of millions to billions of years ago; if faint banding is present, it may be metamorphosed granitic gneiss. Later erosion produced the rounded cobble.
Uses & applications
Used as aggregate, paving stone, landscaping rock, and dimension stone; attractive polished pieces are used decoratively but this specimen has little jewelry value.
Geological facts
Quartz appears glassy or gray, feldspar commonly white, and mica creates tiny reflective flakes. The image does not show enough detail for certain separation of granite from granitic gneiss.
Field identification & locations
Common in continental mountain belts, glacial deposits, riverbeds, and beaches worldwide. Look for interlocking grains, scattered translucent quartz, pale feldspar, and possible dark mica; a streak test is usually white and acid typically causes no reaction.
