metamorphic
Granitic Gneiss
Gneiss (Metamorphic rock predominantly containing quartz, K-feldspar, and plagioclase)
Hardness: 6-7 Mohs; Color: Banded pink, grey, and white; Luster: Variable (dull to vitreous); Structure: Foliated with distinct mineral banding; Cleavage: None (minerals within have individual cleavage); Specific Gravity: 2.6-2.9
- Hardness
- 6-7 Mohs
- Color
- Banded pink, grey, and white
- Luster
- Variable (dull to vitreous)
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Physical properties
Hardness: 6-7 Mohs; Color: Banded pink, grey, and white; Luster: Variable (dull to vitreous); Structure: Foliated with distinct mineral banding; Cleavage: None (minerals within have individual cleavage); Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous granite or sedimentary protoliths. This occurs deep within the Earth's crust under intense heat and pressure, often during mountain-building events (orogenies).
Uses & applications
Used primarily in construction as crushed stone or road base, and as decorative dimension stone for countertops, flooring, and cemetery markers.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The word comes from a Middle High German verb meaning 'to spark'.
Field identification & locations
Identify in the field by looking for 'gneissose banding', where light and dark minerals are segregated into distinct layers. Common in continental shield areas and mountain ranges like the Appalachians or Himalayas.
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