igneous
Biotite-bearing granite or granitic gneiss
Quartz–feldspar–biotite granitoid; exact identification requires testing
AI-generated identification · may be incorrectCoarse-grained, gray and speckled, with glassy gray quartz, pale feldspar, and black platy biotite or amphibole. Typical hardness is about 6–7 Mohs; nonmetallic luster, interlocking crystalline texture, and poor or absent cleavage as a whole.
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Physical properties
Coarse-grained, gray and speckled, with glassy gray quartz, pale feldspar, and black platy biotite or amphibole. Typical hardness is about 6–7 Mohs; nonmetallic luster, interlocking crystalline texture, and poor or absent cleavage as a whole.
Formation & geological history
Likely formed by slow cooling of silica-rich magma deep in the crust, commonly during Precambrian to Phanerozoic plutonic events; if foliation is present, it may be metamorphosed granitic gneiss.
Uses & applications
Commonly used as crushed aggregate, dimension stone, paving, and building material; ordinary hand specimens have little commercial or jewelry value.
Geological facts
The dark minerals are probably biotite mica and/or hornblende, while quartz and feldspar make up most of the rock. A fresh broken surface is more diagnostic than the weathered exterior.
Field identification & locations
Identify it by coarse interlocking grains, abundant light quartz/feldspar, and dark mica or amphibole; granite is widespread worldwide, especially in continental mountain belts. A streak, hardness, acid, and magnetism test can refine identification.
