metamorphic
Gneiss (specifically Granitic Gneiss)
Gneissic Metamorphic Rock
Hardness: 6-7 (Mohs); Color: Pink, orange, gray, and black/green banding; Luster: Vitreous to dull; Structure: Foliated (gneissic banding); Cleavage: None (fractures irregularly); Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs)
- Color
- Pink, orange, gray, and black/green banding
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs); Color: Pink, orange, gray, and black/green banding; Luster: Vitreous to dull; Structure: Foliated (gneissic banding); Cleavage: None (fractures irregularly); Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of a protolith (likely granite or sedimentary rock) under intense heat and pressure deep in the Earth's crust. Often associated with Precambrian or Paleozoic tectonic events.
Uses & applications
Used in construction as dimension stone, crushed stone for road aggregates, architectural flooring, countertops, and occasionally for lapidary work in decorative carving.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The distinctive 'gneissic banding' occurs because minerals segregate into layers during metamorphosis.
Field identification & locations
Identify in the field by looking for 'salt and pepper' appearance with distinct parallel bands or layers of different colors. Commonly found in the crystalline cores of mountain ranges and shield regions like the Canadian Shield or Appalachians.
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