igneous
Feldspar-rich granite or granitic gneiss
Granite; mainly quartz (SiO₂), feldspar, and mica
AI-generated identification · may be incorrectCoarse-grained, gray to dark rock with conspicuous pale feldspar and reflective mica; quartz and feldspar are typically Mohs 6–7, with vitreous to pearly luster and no consistent cleavage as a whole. The specimen appears weathered and possibly foliated, so granitic gneiss is also possible.
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Physical properties
Coarse-grained, gray to dark rock with conspicuous pale feldspar and reflective mica; quartz and feldspar are typically Mohs 6–7, with vitreous to pearly luster and no consistent cleavage as a whole. The specimen appears weathered and possibly foliated, so granitic gneiss is also possible.
Formation & geological history
Granite crystallizes slowly from silica-rich magma deep underground, generally during Precambrian to Phanerozoic mountain-building; gneiss forms when granite is metamorphosed under high heat and pressure.
Uses & applications
Used as crushed aggregate, building stone, countertops, monuments, and decorative rock; ordinary loose specimens have little commercial value.
Geological facts
The light flakes are likely mica or feldspar, while dark grains may be biotite or amphibole. Grain size and mineral interlocking distinguish it from sedimentary rock.
Field identification & locations
Identify it by visible interlocking crystals, abundant light feldspar/quartz, and occasional mica sparkle; a streak test is usually white, and it will not fizz with dilute hydrochloric acid. Common worldwide in continental shields and mountain belts.
