metamorphic
Gneiss
Gneiss
Hardness: 6-7 Mohs; Color: Gray, pink, white, and black; Luster: Vitreous to dull; Crystal structure: Coarse-grained with foliation (gneissose banding); Cleavage: Poorly defined; Specific Gravity: 2.6-3.0
- Hardness
- 6-7 Mohs
- Color
- Gray, pink, white, and black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Gray, pink, white, and black; Luster: Vitreous to dull; Crystal structure: Coarse-grained with foliation (gneissose banding); Cleavage: Poorly defined; Specific Gravity: 2.6-3.0
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary rocks (protoliths) at intense temperatures and pressures deep within the Earth's crust, often during mountain-building events (orogenies).
Uses & applications
Used primarily as crushed stone for road construction and railroad ballast; higher quality specimens are used for architectural facing, countertops, and curbstone.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. It is characterized by alternating bands of light-colored minerals like quartz/feldspar and dark minerals like biotite/amphibole.
Field identification & locations
Identify in the field by look for distinct 'gneissose' banding or foliation. It is commonly found in the core of mountain ranges like the Appalachians, Alps, and Himalayas. Collectors look for unique banding or associated minerals like garnet.
