metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, gray, white, and black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, gray, white, and black; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks like granite or sedimentary rocks like shale. It originates under high temperature and pressure deep within the Earth's crust, often during mountain-building events.
Uses & applications
Used primarily as a dimension stone for flooring, facing, and architectural trim, as well as crushed stone for road construction and landscaping.
Geological facts
Gneiss contains the same minerals as granite but is distinguished by its 'gneissose banding,' where minerals are segregated into dark and light layers. Some of the oldest decorative stones in the world are gneisses.
Field identification & locations
Identified in the field by its distinct alternating bands of light-colored minerals (quartz/feldspar) and dark minerals (biotite/hornblende). It is commonly found in the roots of mountain ranges and ancient continental shields.
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