igneous
Biotite-bearing granite or granitic gneiss
Quartz–feldspar–biotite assemblage; likely granite (possibly weakly metamorphosed gneiss)
AI-generated identification · may be incorrectCoarse-grained, dark gray and speckled with pale feldspar/quartz and black biotite; vitreous quartz and pearly biotite luster, with no obvious cleavage as a whole rock. Individual minerals are typically Mohs 2.5–3 for biotite, 6–7 for feldspar/quartz; specific gravity about 2.6–2.8.
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Physical properties
Coarse-grained, dark gray and speckled with pale feldspar/quartz and black biotite; vitreous quartz and pearly biotite luster, with no obvious cleavage as a whole rock. Individual minerals are typically Mohs 2.5–3 for biotite, 6–7 for feldspar/quartz; specific gravity about 2.6–2.8.
Formation & geological history
Likely crystallized slowly from silica-rich magma deep in the continental crust, commonly during Precambrian to Phanerozoic mountain-building; foliation, if present, would indicate later metamorphism into gneiss.
Uses & applications
Used as crushed aggregate, building stone, curbing, and sometimes countertops or decorative slabs; ordinary hand specimens have modest collector value.
Geological facts
The contrasting dark mica and light quartz–feldspar grains give granite its characteristic “salt-and-pepper” appearance. Biotite commonly splits into thin elastic sheets.
Field identification & locations
Identify it by visible interlocking crystals, abundant pale quartz/feldspar, and flaky black mica; a streak, hardness, and fresh-break test help distinguish it from basalt. Common worldwide in continental plutons and exposed mountain ranges.
