igneous
Rounded granitic cobble
Granite, likely biotite granite or granodiorite
AI-generated identification · may be incorrectMedium- to coarse-grained, speckled gray rock composed mainly of quartz, feldspar, and dark biotite/hornblende; Mohs hardness about 6–7, nonmetallic luster, massive texture, and no cleavage as a whole. Its smooth shape reflects abrasion and weathering.
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Physical properties
Medium- to coarse-grained, speckled gray rock composed mainly of quartz, feldspar, and dark biotite/hornblende; Mohs hardness about 6–7, nonmetallic luster, massive texture, and no cleavage as a whole. Its smooth shape reflects abrasion and weathering.
Formation & geological history
An intrusive igneous rock crystallized slowly from silica-rich magma deep in the crust, commonly during Precambrian to Mesozoic mountain-building events; this specimen was later transported or weathered into a rounded cobble.
Uses & applications
Used for crushed aggregate, building stone, curbs, monuments, countertops, and landscaping; ordinary cobbles have little commercial value.
Geological facts
Granite’s visible grains indicate slow underground cooling, while its rounded form indicates long-term erosion, stream transport, or glacial movement.
Field identification & locations
Identify it by interlocking light quartz/feldspar grains with scattered black mica or amphibole; a glassy quartz-rich surface and lack of layering distinguish it from sedimentary rocks. Common worldwide, especially in continental mountain belts.
