metamorphic
Gneiss
Gneiss
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and pink; Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissic banding); Specific Gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded grey, white, and pink
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and pink; Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissic banding); Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) under extreme pressure and temperature. Foundations of continents (cratons) often contain Proterozoic gneiss.
Uses & applications
Used in construction as crushed stone or dimension stone for countertops, flooring, and landscaping. Occasionally used in jewelry if it contains decorative minerals like garnets.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known crustal rocks on Earth, dated to approximately 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' or 'to glisten.'
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating layers of light (felsic) and dark (mafic) minerals. Common in mountain ranges and glacial deposits. Avoid confusing with schist, which has finer layers and more mica.
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