metamorphic
Gneiss (specifically Granitic Gneiss)
Gneiss (composed primarily of Quartz, Feldspar, and Biotite/Amphibole)
Hardness: 6-7 on Mohs scale; Color: Pink, white, and black banded patterns; Luster: Vitreous to dull; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Usually none in the rock itself, though individual mineral grains may have it; Specific Gravity: 2.6–2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, white, and black banded patterns
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, white, and black banded patterns; Luster: Vitreous to dull; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Usually none in the rock itself, though individual mineral grains may have it; Specific Gravity: 2.6–2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary rocks under high temperature and pressure. The banding occurs as minerals recrystallize and segregate. Most specimens are pre-cambrian or paleozoic in age.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases or as 'granite' for countertops, flooring, and ornamental building stone.
Geological facts
Gneiss is some of the oldest rock found on Earth’s surface; the Acasta Gneiss in Canada is approximately 4.03 billion years old. The bands are generally perpendicular to the direction of the pressure applied during formation.
Field identification & locations
Identify in the field by looking for 'Gneissic Banding'—distinct alternating layers of light-colored minerals (quartz/feldspar) and dark-colored minerals (biotite/hornblende). It is commonly found in the cores of mountain ranges and glacial till across North America and Scandinavia.
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