igneous
Coarse-grained granite or granitic gneiss
Granite; primarily quartz (SiO₂), feldspar, and mica/amphibole
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray-black and white, with a coarse crystalline texture and vitreous to pearly mineral luster. Interlocking quartz and feldspar grains are visible; foliation is possible if this is gneiss.
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Physical properties
Hardness about 6–7 Mohs; gray-black and white, with a coarse crystalline texture and vitreous to pearly mineral luster. Interlocking quartz and feldspar grains are visible; foliation is possible if this is gneiss.
Formation & geological history
Granite crystallizes slowly from silica-rich magma deep in the crust, typically during Precambrian to younger continental plutonic events; gneiss forms when granite is metamorphosed under high temperature and pressure.
Uses & applications
Used as dimension stone, countertops, monuments, crushed aggregate, and building material; attractive hand specimens have modest collecting value.
Geological facts
The dark grains are likely biotite mica or hornblende, while pale grains are feldspar and quartz. A strong compositional banding would favor granitic gneiss over granite.
Field identification & locations
Common worldwide in continental shields and mountain belts, including the Appalachian, Canadian, Scandinavian, and Australian regions. Identify by its visible interlocking crystals, lack of vesicles, and inability to scratch easily with a steel knife.
