igneous
Biotite-bearing granite or granitic gneiss
Quartz–feldspar–biotite granitoid; exact identification requires testing
AI-generated identification · may be incorrectCoarse-grained, gray-and-white rock with shiny black biotite mica and likely quartz/feldspar; vitreous to pearly luster, interlocking crystals, Mohs hardness about 6–7, poor cleavage as a rock, and density roughly 2.6–2.8 g/cm³.
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Physical properties
Coarse-grained, gray-and-white rock with shiny black biotite mica and likely quartz/feldspar; vitreous to pearly luster, interlocking crystals, Mohs hardness about 6–7, poor cleavage as a rock, and density roughly 2.6–2.8 g/cm³.
Formation & geological history
Likely crystallized slowly from silica-rich magma deep in the crust, or represents metamorphosed granite if faint foliation is present; commonly Precambrian to Phanerozoic in age.
Uses & applications
Used as crushed stone, building and architectural stone, countertops, monuments, and aggregate; small unattractive pieces have minimal jewelry value.
Geological facts
Black reflective flakes are characteristic of biotite, while translucent gray grains may be quartz and pale grains feldspar. A streak test, hand lens, and fresh fracture would improve identification.
Field identification & locations
Common in continental crust and exposed in mountain belts worldwide, including the United States, Canada, Scandinavia, and India. Look for visible quartz, blocky feldspar, and flexible dark mica flakes; strong layering would favor gneiss.
