metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Banded pink, white, and dark grey; Luster: Dull to pearly; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined; Specific gravity: 2.6-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded pink, white, and dark grey
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded pink, white, and dark grey; Luster: Dull to pearly; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined; Specific gravity: 2.6-3.0.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) under intense heat and pressure deep within the Earth's crust.
Uses & applications
Commonly used as a decorative building stone, for flooring, gravestones, and as crushed stone for road construction and landscaping.
Geological facts
The word 'gneiss' comes from an Old High German verb meaning 'to spark' or 'to glisten.' It is among the oldest known rocks on Earth, with some specimens dating back over 4 billion years.
Field identification & locations
Identified in the field by its characteristic alternating bands of light-colored minerals (quartz/feldspar) and dark-colored minerals (biotite/amphibole). It is found globally in the cores of mountain ranges and ancient continental shields.
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