igneous
Biotite-bearing granite or granodiorite
Coarse-grained felsic intrusive rock; chiefly quartz, feldspar, and biotite mica
AI-generated identification · may be incorrectGray, speckled, granular texture with black reflective mica and pale feldspar/quartz; Mohs hardness about 6–7 overall, vitreous to pearly luster, blocky feldspar cleavage, no layering. Exact identification is limited by the photograph and weathered surface.
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Physical properties
Gray, speckled, granular texture with black reflective mica and pale feldspar/quartz; Mohs hardness about 6–7 overall, vitreous to pearly luster, blocky feldspar cleavage, no layering. Exact identification is limited by the photograph and weathered surface.
Formation & geological history
Formed when silica-rich magma cooled slowly deep underground, producing visible interlocking crystals; commonly Precambrian to Phanerozoic in age, depending on locality.
Uses & applications
Used as crushed aggregate, road stone, building stone, countertops, monuments, and landscaping material; ordinary hand specimens have little jewelry value.
Geological facts
The black flakes are likely biotite, which splits into thin elastic sheets and often gives granite its distinctive sparkle. Granodiorite contains more plagioclase than potassium feldspar.
Field identification & locations
Look for interlocking visible crystals, abundant light quartz/feldspar, and shiny black mica flakes; it is common in continental mountain belts and exposed plutons worldwide. A streak, hardness test, and fresh broken surface would improve identification.
