metamorphic
Gneiss
Metamorphic Gneiss (High-grade regional metamorphism)
Hardness: 6-7 Mohs; Color: Gray, black, and white with banded texture; Luster: Variable (pearly to dull); Structure: Foliated with distinct bands (gneissic banding); Cleavage: Poor, typically breaks along grain boundaries.
- Hardness
- 6-7 Mohs
- Color
- Gray, black, and white with banded texture
- Luster
- Variable (pearly to dull)
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Physical properties
Hardness: 6-7 Mohs; Color: Gray, black, and white with banded texture; Luster: Variable (pearly to dull); Structure: Foliated with distinct bands (gneissic banding); Cleavage: Poor, typically breaks along grain boundaries.
Formation & geological history
Formed through high-grade regional metamorphism where pre-existing igneous or sedimentary rocks are subjected to intense heat and pressure, causing recrystallization and segregation of minerals into bands.
Uses & applications
Commonly used as a decorative stone in architecture, countertops, and flooring, or as crushed stone in road construction and concrete aggregate.
Geological facts
Gneiss is one of the oldest rocks found on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The word originates from the Middle High German verb 'gneist' meaning 'to spark'.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating layers of light minerals like quartz or feldspar and dark minerals like biotite or hornblende. Widely distributed in crystalline basement rocks globally.
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