metamorphic
Gneiss
Foliated Metamorphic Rock
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white bands; Luster: Vitreous to dull; Structure: Coarse-grained foliated (gneissic banding); Cleavage: Poor, usually breaks along uneven planes; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, and white bands
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white bands; Luster: Vitreous to dull; Structure: Coarse-grained foliated (gneissic banding); Cleavage: Poor, usually breaks along uneven planes; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under intense heat and pressure, often during mountain-building events (orogenies) dating from the Precambrian to Cenezoic eras.
Uses & applications
Used primarily as crushed stone for road construction and concrete aggregate, also used as dimension stone for building facades, flooring, and cemetery monuments.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada estimated to be over 4 billion years old. The term comes from an Old High German word meaning 'to sparkle.'
Field identification & locations
Identify in the field by looking for 'gneissic banding,' which is the segregation of light (quartz/feldspar) and dark (biotite/amphibole) minerals into distinct layers. Found globally in the cores of mountain ranges and ancient continental shields.
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