igneous
Porphyritic granite pebble
Granite, chiefly quartz (SiO₂), alkali feldspar and plagioclase, with minor biotite/hornblende
AI-generated identification · may be incorrectHardness about 6–7 Mohs; light gray-white with black mineral specks, granular texture, and vitreous to dull luster. Interlocking crystals are visible, including larger feldspar grains in a finer matrix; no obvious cleavage as a whole rock.
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Physical properties
Hardness about 6–7 Mohs; light gray-white with black mineral specks, granular texture, and vitreous to dull luster. Interlocking crystals are visible, including larger feldspar grains in a finer matrix; no obvious cleavage as a whole rock.
Formation & geological history
Intrusive igneous rock formed by slow cooling of silica-rich magma deep in the crust, commonly during Precambrian to Phanerozoic mountain-building events. Its rounded shape indicates later erosion and transport by water or glacial action.
Uses & applications
Used widely as crushed aggregate, building stone, countertops, monuments, and decorative landscaping rock; small attractive pebbles are collected but have little gem value.
Geological facts
Granite’s visible grains record slow underground cooling, unlike volcanic rhyolite, which cools rapidly. Dark flecks are typically biotite mica or amphibole, while pale crystals are feldspars and quartz.
Field identification & locations
Identify it by its coarse interlocking grains, pale color, and mixed black-white mineral pattern; a glassy surface or uniform fine grain would suggest another material. Common worldwide in continental mountain belts and glacial/alluvial deposits.
