metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 (Mohs scale), Color: Banded or mottled gray, pink, white, and black. Luster: Dull to vitreous. Crystal structure: Foliated (gneissose banding). Cleavage: Poorly defined. Specific gravity: 2.6-3.0.
- Color
- Banded or mottled gray, pink, white, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Banded or mottled gray, pink, white, and black. Luster: Dull to vitreous. Crystal structure: Foliated (gneissose banding). Cleavage: Poorly defined. Specific gravity: 2.6-3.0.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary rocks (shale) at high temperatures and pressures deep within the Earth's crust, often during mountain-building events.
Uses & applications
Used primarily as crushed stone for road construction, railroad ballast, and building stone. Highly decorative slabs are used for countertops and flooring.
Geological facts
Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada which is approximately 4 billion years old. The banding occurs because minerals separate into layers under extreme pressure.
Field identification & locations
Identify in the field by its distinct alternating bands of light-colored minerals (quartz/feldspar) and dark-colored minerals (biotite/amphibole). It is common in the cores of mountain ranges and glacial deposits.
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