metamorphic
Banded Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 (Mohs scale); Color: Black, white, and gray striped; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-3.0.
- Hardness
- 6-7 (Mohs scale)
- Color
- Black, white, and gray striped
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Black, white, and gray striped; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-3.0.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/mudstone) protoliths under extreme heat and pressure. Most gneiss is Precambrian or Paleozoic in age, formed in the roots of ancient mountain ranges.
Uses & applications
Primarily used as crushed stone for road construction and concrete aggregate. High-quality slabs are used for countertops, floor tiles, and as ornamental building stone.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The bands form because minerals segregate into layers during intense tectonic deformation.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating layers of light minerals (quartz, feldspar) and dark minerals (biotite, amphibole). It lacks the easy splitting (cleavage) found in schist.
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