metamorphic
Gneissic granite / granitic gneiss
Quartz–feldspar–mica gneiss; composition commonly quartz (SiO₂), feldspars, and biotite/muscovite
AI-generated identification · may be incorrectGray-brown, coarse-grained, weakly banded and sparkly from mica; quartz and feldspar have a vitreous to pearly luster. Hardness is approximately 6–7 Mohs, with no obvious cleavage as a whole and uneven fracture; typical specific gravity is 2.6–2.8.
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Physical properties
Gray-brown, coarse-grained, weakly banded and sparkly from mica; quartz and feldspar have a vitreous to pearly luster. Hardness is approximately 6–7 Mohs, with no obvious cleavage as a whole and uneven fracture; typical specific gravity is 2.6–2.8.
Formation & geological history
Formed when granite or another felsic protolith underwent high-grade regional metamorphism, recrystallizing and aligning minerals into foliation. Most examples are Precambrian to Paleozoic, though age depends on locality.
Uses & applications
Used as crushed aggregate, road stone, building stone, countertops, landscaping rock, and occasional decorative slabs; this rounded specimen has modest collector value.
Geological facts
The glittering flakes are likely mica, while pale grains are quartz and feldspar. Foliation or subtle compositional banding distinguishes gneiss from ordinary granite.
Field identification & locations
Common in ancient continental shields and metamorphic mountain belts, including the Canadian Shield, Scandinavia, India, Brazil, and the United States. Look for alternating light/dark mineral bands, abundant quartz/feldspar, and a scratch resistance near that of glass.
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