igneous
Biotite-bearing granite or granitic gneiss
Granite; primarily quartz (SiO₂), feldspar, and biotite mica
AI-generated identification · may be incorrectMedium- to coarse-grained, gray rock with glittering dark mica and pale feldspar/quartz; hardness about 6–7 Mohs, vitreous to pearly luster, interlocking crystalline texture, and generally no cleavage as a whole (mica has perfect cleavage).
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Physical properties
Medium- to coarse-grained, gray rock with glittering dark mica and pale feldspar/quartz; hardness about 6–7 Mohs, vitreous to pearly luster, interlocking crystalline texture, and generally no cleavage as a whole (mica has perfect cleavage).
Formation & geological history
Likely crystallized slowly from silica-rich magma deep in the crust, or represents metamorphosed granite if the faint banding is genuine. Such rocks commonly formed during Precambrian to younger continental mountain-building events.
Uses & applications
Used as crushed aggregate, building stone, monuments, countertops, and decorative slabs; this weathered hand specimen has little commercial or collector value.
Geological facts
Quartz appears glassy or gray, feldspar is white to pink, and biotite produces the conspicuous black reflective flakes. Gneiss typically shows compositional banding or mineral alignment, whereas granite is more uniformly speckled.
Field identification & locations
Identify it by its coarse interlocking grains, abundant light quartz/feldspar, and black mica; it is not likely a gemstone. Common worldwide in continental shields, mountain belts, and granite quarries.
