igneous
Biotite-bearing granite or granodiorite
Coarse-grained quartz–feldspar rock with biotite; approximate composition: quartz + plagioclase/K-feldspar + biotite ± hornblende
AI-generated identification · may be incorrectHardness about 6–7 overall; light gray to pink, with glassy gray quartz, pale feldspar, and reflective black biotite. Granular, crystalline texture; feldspar cleavage is visible, while quartz has none; typical specific gravity is 2.6–2.8.
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Physical properties
Hardness about 6–7 overall; light gray to pink, with glassy gray quartz, pale feldspar, and reflective black biotite. Granular, crystalline texture; feldspar cleavage is visible, while quartz has none; typical specific gravity is 2.6–2.8.
Formation & geological history
Formed when silica-rich magma cooled slowly deep underground, allowing large interlocking crystals to grow; commonly Precambrian to Mesozoic in age depending on locality. Weathering has produced some rusty iron-oxide staining.
Uses & applications
Widely used as dimension stone, building aggregate, curbstone, monuments, and countertops; attractive pieces are collected but usually have low commercial value.
Geological facts
The black flakes are likely biotite mica, whose perfect basal cleavage creates shiny sheets. Pink areas generally indicate potassium feldspar, while translucent gray grains are quartz.
Field identification & locations
Identify it by its coarse interlocking grains and absence of layering or foliation; unlike volcanic rhyolite, crystals are readily visible. Common in continental mountain belts and granite batholiths worldwide.
