igneous
Biotite granite (weathered granitic rock)
Granite, chiefly quartz (SiO₂), feldspar, and biotite mica
AI-generated identification · may be incorrectCoarse-grained, speckled tan, white, gray, and black; vitreous quartz, pale feldspar, and flaky dark biotite. Mohs hardness about 6–7; massive, nonfoliated, with no dominant cleavage as a whole; specific gravity roughly 2.6–2.7.
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Physical properties
Coarse-grained, speckled tan, white, gray, and black; vitreous quartz, pale feldspar, and flaky dark biotite. Mohs hardness about 6–7; massive, nonfoliated, with no dominant cleavage as a whole; specific gravity roughly 2.6–2.7.
Formation & geological history
Slow cooling of silica-rich magma deep within continental crust allows large crystals to form; commonly Precambrian to Phanerozoic in age. The rounded edges and surface staining indicate weathering or water abrasion.
Uses & applications
Widely used as dimension stone, countertops, monuments, aggregate, and building material; attractive pieces are collected but usually have modest gem value.
Geological facts
Granite’s visible interlocking crystals record prolonged underground cooling. Black biotite commonly distinguishes it from lighter quartz-rich rocks such as quartzite.
Field identification & locations
Identify by coarse interlocking crystals, abundant pale feldspar, translucent gray quartz, and black mica; it should not fizz with dilute hydrochloric acid. Common worldwide in continental mountain belts and exposed plutons.
