metamorphic
Granitic Gneiss
Gneiss (Complex silicate composition)
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, pink, and black; Luster: Dull to vitreous; Structure: Foliated with distinct mineral banding; Cleavage: Poor; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, white, pink, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, pink, and black; Luster: Dull to vitreous; Structure: Foliated with distinct mineral banding; Cleavage: Poor; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous protoliths (like granite) or sedimentary rocks. This occurs under intense pressure and temperature deep within the Earth's crust, often during mountain-building events (orogenies) over millions of years.
Uses & applications
Used primarily as a dimension stone for flooring, kitchen countertops, and gravestones. It is also crushed for use as aggregate in road construction and landscaping.
Geological facts
Gneiss contains some of the oldest known rock on Earth, such as the Acasta Gneiss in Canada which is roughly 4 billion years old. The banding is caused by the segregation of light minerals like quartz/feldspar and dark minerals like biotite/hornblende.
Field identification & locations
Identified in the field by its 'gneissose banding'—alternating layers of light and dark minerals. It will not fizz with acid (unlike marble). Commonly found in the crystalline cores of mountain ranges and ancient continental shields.
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