metamorphic
Gneiss (Metamorphic Rock)
Gneiss
Hardness: 6-7 on Mohs scale. Color: Gray, pink, white, and black banded patterns. Luster: Vitreous to dull. Structure: Foliated with distinct bands (gneissose banding). Cleavage: Generally poor, breaks along blocky joints.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, and black banded patterns
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Gray, pink, white, and black banded patterns. Luster: Vitreous to dull. Structure: Foliated with distinct bands (gneissose banding). Cleavage: Generally poor, breaks along blocky joints.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale/mudstone). Subjected to high temperatures and pressures deep within the Earth's crust, typically during mountain-building events (orogenies).
Uses & applications
Used as a decorative building stone, crushed stone for road construction and railroad ballast, and occasionally for landscaping or as a polishing material in rock tumblers.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with the Acasta Gneiss in Canada being approximately 4 billion years old. The term 'gneiss' comes from an old German word meaning 'sparkling' or 'to glow'.
Field identification & locations
Identify in the field by its characteristic alternating bands of light-colored (felsic) minerals like quartz/feldspar and dark-colored (mafic) minerals like biotite/hornblende. Commonly found in the crystalline cores of mountain ranges.
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