igneous
Biotite-bearing granite or granitic gneiss
Quartz–feldspar–mica rock; chiefly quartz (SiO₂), feldspars, and biotite mica
AI-generated identification · may be incorrectCoarse, speckled texture with pale quartz/feldspar and dark, flaky biotite; likely Mohs 6–7 overall, vitreous to pearly luster, no obvious cleavage as a whole rock, and specific gravity about 2.6–2.8.
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Physical properties
Coarse, speckled texture with pale quartz/feldspar and dark, flaky biotite; likely Mohs 6–7 overall, vitreous to pearly luster, no obvious cleavage as a whole rock, and specific gravity about 2.6–2.8.
Formation & geological history
Likely crystallized slowly from silica-rich magma deep underground, commonly during Precambrian to younger continental crust formation; foliation, if present, would indicate later metamorphism into gneiss.
Uses & applications
Used as aggregate, building stone, monuments, and decorative material; small irregular pieces have minimal jewelry value unless unusually attractive or historically significant.
Geological facts
The dark minerals are probably biotite and/or hornblende, while translucent gray grains are quartz. A strong planar banding would favor granitic gneiss over granite.
Field identification & locations
Common in continental mountain belts and exposed basement rocks worldwide, including North America, Europe, India, and Australia. Identify with a hand lens: quartz is glassy and lacks cleavage, feldspar is commonly blocky with cleavage, and biotite splits into thin dark sheets.
