metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banded; Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissic banding); Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, and black banded
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banded; 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 intense heat and pressure, typically during mountain-building events.
Uses & applications
Used as crushed stone for road construction, paving, and landscaping. High-quality slabs are used for countertops, floor tiles, and building veneers.
Geological facts
Gneiss can be among the oldest rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. The term comes from an old German word meaning 'sparkling'.
Field identification & locations
Identify by looking for 'gneissic banding'—alternating layers of light minerals (quartz, feldspar) and dark minerals (biotite, amphibole). Found globally in the cores of mountain ranges and ancient continental shields.
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