igneous
Biotite-bearing granite (possibly granodiorite)
Coarse crystalline quartz–feldspar–biotite rock; approximate composition: quartz SiO₂, feldspars, and biotite K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂
AI-generated identification · may be incorrectHardness about 6–7 overall; gray to black and white, with a granular texture and vitreous-to-pearly mineral luster. Interlocking crystals show no obvious foliation or cleavage at the rock scale; specific gravity is roughly 2.6–2.8.
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Physical properties
Hardness about 6–7 overall; gray to black and white, with a granular texture and vitreous-to-pearly mineral luster. Interlocking crystals show no obvious foliation or cleavage at the rock scale; specific gravity is roughly 2.6–2.8.
Formation & geological history
A plutonic igneous rock crystallized slowly from silica-rich magma deep in the crust, producing visible crystals; its exact geological age cannot be determined from the photograph and varies by locality.
Uses & applications
Used as dimension stone, countertops, monuments, aggregate, and landscaping rock; attractive polished pieces may be collected but are usually inexpensive.
Geological facts
The dark flakes are likely biotite or hornblende, while pale grains are feldspar and reflective gray grains may be quartz. The rock could be granodiorite if plagioclase dominates over potassium feldspar.
Field identification & locations
Identify it by coarse interlocking grains, abundant pale feldspar/quartz, and black mica or amphibole; a glassy fracture, acid test, and thin-section analysis can refine identification. Common worldwide in continental plutons and mountain belts.
