metamorphic
Gneiss
Gneiss (Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Gray, black, pink, or white, typically banded; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated texture with distinct mineral segregation; Cleavage: Generally poor, breaks along bands.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, pink, or white, typically banded
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, pink, or white, typically banded; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated texture with distinct mineral segregation; Cleavage: Generally poor, breaks along bands.
Formation & geological history
Formed by the high-grade regional metamorphism of pre-existing sedimentary (paragneiss) or igneous rocks (orthogneiss). It undergoes intense heat and pressure, causing minerals to recrystallize and segregate into alternating light and dark layers.
Uses & applications
Used as a dimension stone for flooring, ornamental building facades, gravestones, and crushed stone for road construction and landscaping.
Geological facts
Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada which is roughly 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' or 'to glisten'.
Field identification & locations
Identified by 'gneissose banding' (alternating light and dark layers). Look for prominent crystals of quartz, feldspar, and dark minerals like biotite. It is extremely common in the roots of mountain ranges and ancient continental shields.
