igneous
Rounded granitic gneiss or granite cobble
Granite; primarily quartz, plagioclase and potassium feldspar with biotite/hornblende
AI-generated identification · may be incorrectMedium-grained, gray and speckled with a granular crystalline texture; likely Mohs 6–7, vitreous-to-dull luster, no obvious cleavage as a whole, and specific gravity about 2.6–2.8. The rounded shape indicates natural abrasion or tumbling.
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Physical properties
Medium-grained, gray and speckled with a granular crystalline texture; likely Mohs 6–7, vitreous-to-dull luster, no obvious cleavage as a whole, and specific gravity about 2.6–2.8. The rounded shape indicates natural abrasion or tumbling.
Formation & geological history
Formed by slow crystallization of silica-rich magma deep in the crust, generally during Precambrian to Phanerozoic mountain-building events; the specimen was later weathered and rounded in a river or shoreline environment.
Uses & applications
Widely used as crushed stone, aggregate, building stone, monuments and landscaping material; ordinary cobbles have little jewelry value.
Geological facts
Quartz grains appear glassy or gray, feldspar grains are pale, and dark flecks are commonly biotite or hornblende. The image does not show clear banding, so granite is more likely than gneiss.
Field identification & locations
Identify it by visible interlocking grains and the mix of light quartz/feldspar with dark mica or amphibole; it is not a single crystal. Common worldwide, especially in continental shields and mountain belts.
