igneous
Biotite-bearing granite or granodiorite
Coarse-grained felsic intrusive rock; primarily quartz (SiO₂), feldspar, and biotite mica, possibly with hornblende
AI-generated identification · may be incorrectHardness about 6–7 overall; mottled black, gray, white, and brown, with a granular texture and mostly vitreous to dull luster. Interlocking crystals are visible, with little or no cleavage at the rock scale; density is typically 2.6–2.8 g/cm³.
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Physical properties
Hardness about 6–7 overall; mottled black, gray, white, and brown, with a granular texture and mostly vitreous to dull luster. Interlocking crystals are visible, with little or no cleavage at the rock scale; density is typically 2.6–2.8 g/cm³.
Formation & geological history
Formed when silica-rich magma cooled slowly deep underground, allowing large crystals to grow; commonly Precambrian to Mesozoic in age depending on locality. The photograph does not permit reliable separation of granite from granodiorite.
Uses & applications
Used as dimension stone, countertops, monuments, aggregate, and landscaping rock; attractive polished pieces may be collected but are generally inexpensive.
Geological facts
Dark flakes are likely biotite or hornblende, while pale grains are feldspar and quartz. A glassy quartz grain, hardness near 7, and lack of layering support an intrusive igneous identification.
Field identification & locations
Common worldwide, including the Sierra Nevada, Canadian Shield, Appalachians, and Scandinavian Shield. Identify it by its coarse interlocking grains, lack of foliation, and inability of a steel knife to scratch quartz-rich grains.
