metamorphic
Gneiss (with Quartzite and Mica)
Gneiss
Hardness: 6-7 (Mohs scale); Color: Brown, tan, gray; Luster: Vitreous to dull; Crystal structure: Coarse-grained crystalline; Cleavage: Poorly defined, exhibits gneissic banding; Specific gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Brown, tan, gray
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Brown, tan, gray; Luster: Vitreous to dull; Crystal structure: Coarse-grained crystalline; Cleavage: Poorly defined, exhibits gneissic banding; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths. The banding occurs when the rock is subjected to high temperatures and pressures deep within the Earth's crust, causing minerals to segregate into layers.
Uses & applications
Used primarily as a dimension stone for flooring, cemetery markers, facing on buildings, and as crushed stone for road construction and railroad ballast.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. The word 'gneiss' comes from an old German mining term meaning 'sparkling' due to the presence of mica.
Field identification & locations
Identify in the field by looking for distinct 'foliation' or mineral banding where dark and light minerals are separated into stripes. It will not easily crumble and often contains visible flecks of shiny mica or translucent quartz. Common in mountain ranges and crystalline shield areas.
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