metamorphic
Gneiss
Gneiss (Metamorphic rock composed primarily of quartz, feldspar, and mica)
Hardness: 6-7 (Mohs scale); Color: Gray, pink, white, or black with bands; Luster: Vitreous to dull; Structure: Foliated with distinct banding (gneissose banding); Specific Gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray, pink, white, or black with bands
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray, pink, white, or black with bands; Luster: Vitreous to dull; Structure: Foliated with distinct banding (gneissose banding); Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/mudstone) rocks at high temperatures and pressures. Most common in Precambrian shields.
Uses & applications
Used in construction as a building stone, paving blocks, crushed stone for road base, and occasionally as decorative stone or countertops.
Geological facts
Gneiss includes some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old.
Field identification & locations
Identify by looking for 'gneissose banding'—distinct layered appearances where dark and light minerals separate into stripes. Found globally in mountain cores and shields.
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