igneous
Porphyritic granite
Coarse-grained felsic intrusive igneous rock, chiefly quartz, potassium feldspar, plagioclase, and biotite/hornblende
AI-generated identification · may be incorrectHardness about 6–7; dark gray to black matrix with conspicuous pale feldspar crystals and minor glassy quartz. Granular texture, vitreous to dull luster, no prominent cleavage as a rock, and specific gravity about 2.6–2.8.
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Physical properties
Hardness about 6–7; dark gray to black matrix with conspicuous pale feldspar crystals and minor glassy quartz. Granular texture, vitreous to dull luster, no prominent cleavage as a rock, and specific gravity about 2.6–2.8.
Formation & geological history
Magma cooled slowly deep within the crust, allowing large crystals to form; likely Precambrian to Phanerozoic, though age cannot be determined from the photograph.
Uses & applications
Used as dimension stone, countertops, paving aggregate, monuments, and landscaping rock; attractive specimens are collected but usually have modest value.
Geological facts
The large pale crystals are phenocrysts or megacrysts, recording an early stage of crystallization before the remaining magma solidified.
Field identification & locations
Identify it by its interlocking crystalline texture, abundant light feldspar, and darker biotite or hornblende. Common worldwide in continental mountain belts and exposed plutons; a hand lens and fresh broken surface help distinguish it from gneiss.
