metamorphic
Granite Gneiss
Granitoid Gneiss
Hardness: 6-7 on Mohs scale. Color: Pink, white, gray, and black mottled patterns with distinct foliation or banding. Luster: Dull to vitreous (glassy quartz grains). Texture: Medium to coarse-grained metamorphic texture. Specific Gravity: ~2.6-2.7.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous (glassy quartz grains)
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Pink, white, gray, and black mottled patterns with distinct foliation or banding. Luster: Dull to vitreous (glassy quartz grains). Texture: Medium to coarse-grained metamorphic texture. Specific Gravity: ~2.6-2.7.
Formation & geological history
Formed under high-temperature and high-pressure regional metamorphism of igneous granite or sedimentary protoliths, typically in deep continental crust during mountain-building events (orogenies).
Uses & applications
Used in construction as dimension stone, crushed stone for road base, architectural facing, countertops, landscaping rock, and decorative specimens.
Geological facts
Gneiss exhibits characteristically distinct dark and light bands called gneissic foliation, formed by the segregation of mafic (biotite/amphibole) and felsic (quartz/feldspar) minerals during extreme metamorphism without melting.
Field identification & locations
Identified by alternating light (pink potassium feldspar and quartz) and dark (biotite/hornblende) mineral bands. Commonly found in ancient continental shields, mountain ranges (e.g., Appalachians, Alps, Rockies), and glaciated beach cobbles.
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