metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Gray, black, white, pinkish; Luster: Pearly to dull; Structure: Foliated with distinct bands (gneissic banding); Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, white, pinkish
- Luster
- Pearly to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, white, pinkish; Luster: Pearly to dull; Structure: Foliated with distinct bands (gneissic banding); Specific Gravity: 2.6-2.9
Formation & geological history
Formed by high-pressure, high-temperature regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths). It represents one of the highest grades of metamorphism.
Uses & applications
Extensively used in construction as crushed stone for road bases, and as dimension stone for building facades, floor tiles, and gravestones.
Geological facts
Gneiss contains some of the oldest known rock on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old.
Field identification & locations
Identified in the field by its distinct alternating layers of light-colored minerals (quartz/feldspar) and dark-colored minerals (biotite/amphibole). It is common in the roots of mountain ranges and ancient continental shields.
