igneous
Biotite-bearing granite or granitic gneiss
Quartz–feldspar–biotite rock; exact identification requires testing
AI-generated identification · may be incorrectCoarse-grained, dark gray and speckled, with glassy gray quartz, pale feldspar, and black flaky biotite mica. Quartz is Mohs 7, feldspar about 6, and biotite 2.5–3; luster is vitreous to pearly, with no obvious cleavage in the whole rock.
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Physical properties
Coarse-grained, dark gray and speckled, with glassy gray quartz, pale feldspar, and black flaky biotite mica. Quartz is Mohs 7, feldspar about 6, and biotite 2.5–3; luster is vitreous to pearly, with no obvious cleavage in the whole rock.
Formation & geological history
Likely crystallized slowly from silica-rich magma deep in the crust, or represents deformed equivalent gneiss; typical ages range from hundreds of millions to billions of years. The visible interlocking crystals indicate slow cooling.
Uses & applications
Common granitic rocks are used as crushed aggregate, building stone, countertops, monuments, and landscaping material; attractive pieces may be collected but are generally not gemstone-grade.
Geological facts
The dark minerals may also include hornblende, and the apparent layering could indicate gneiss rather than granite. A streak test, acid test, and close examination of foliation would refine the identification.
Field identification & locations
Common worldwide, especially in continental mountain belts and exposed crystalline shields. Identify it by interlocking visible crystals, abundant quartz/feldspar, and black mica flakes; granite is massive, whereas gneiss shows compositional banding.
