metamorphic
Gneiss
Metamorphic Gneiss (High-grade regional metamorphism)
Hardness: 6-7 Mohs; Color: Banded gray, black, and tan/white; Luster: Dull to sub-vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Poorly developed along bands.
- Hardness
- 6-7 Mohs
- Color
- Banded gray, black, and tan/white
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Banded gray, black, and tan/white; Luster: Dull to sub-vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Poorly developed along bands.
Formation & geological history
Formed by high-pressure, high-temperature regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale). Often dates back billions of years to Precambrian ages in shield areas.
Uses & applications
Used frequently in construction as crushed stone for road bases or as architectural dimension stone for countertops and tiling due to its durability.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. The banding occurs because minerals segregate during intense geological heating and squeezing.
Field identification & locations
Identify by looking for distinct layers or bands of different colored minerals (usually quartz, feldspar, and mica). It is common in mountainous regions and glacial deposits.
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