igneous
Biotite-rich granite or gneiss fragment
Dark mica-bearing granitoid; likely quartz–feldspar–biotite rock
AI-generated identification · may be incorrectDark gray to black, coarse-grained, with reflective platy biotite and possible quartz/feldspar; vitreous to pearly luster. Typical hardness is about 2.5–3 for biotite and 6–7 for quartz/feldspar; cleavage is strong in mica, with no visible crystal shape at this scale.
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Physical properties
Dark gray to black, coarse-grained, with reflective platy biotite and possible quartz/feldspar; vitreous to pearly luster. Typical hardness is about 2.5–3 for biotite and 6–7 for quartz/feldspar; cleavage is strong in mica, with no visible crystal shape at this scale.
Formation & geological history
Likely formed by slow cooling of silica-rich magma, or represents metamorphosed granite if foliation is present; commonly Precambrian to Phanerozoic in age. The image alone cannot distinguish granite from gneiss confidently.
Uses & applications
Used as crushed stone, aggregate, dimension stone, and building material; attractive polished pieces may be collected, but this fragment has little commercial value.
Geological facts
The glittering surfaces are probably biotite mica, whose perfect cleavage allows it to split into flexible sheets. Dark appearance can also result from hornblende or abundant magnetite.
Field identification & locations
Common in continental crust and found worldwide, especially in mountain belts and granite quarries. Confirm with a hand lens: visible quartz, pink/white feldspar, and black mica suggest granite; aligned mineral bands suggest gneiss.
