metamorphic
Gneiss (specifically a Granitic Gneiss)
Gneiss (Composition: Quartz, Feldspar, Mica)
Hardness: 6-7 on Mohs scale; Color: Banded pink, white, and black; Luster: Vitreous to dull; Structure: Gneissose foliation (distinct layering); Cleavage: Poor, but breaks along foliation; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded pink, white, and black
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded pink, white, and black; Luster: Vitreous to dull; Structure: Gneissose foliation (distinct layering); Cleavage: Poor, but breaks along foliation; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of granite or sedimentary protoliths. This process occurs under extreme heat and pressure typical of mountain-building events (orogeny). Age can range from Pre-Cambrian to more recent tectonic periods.
Uses & applications
Primarily used in construction as crushed stone for road bases or as dimension stone for counter-tops, flooring, and architectural facing. High-quality specimens are collected for educational and geological interest.
Geological facts
Gneiss contains some of the oldest known crustal rocks on Earth, such as the Acasta Gneiss which is approximately 4.03 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' because the rock glitters.
Field identification & locations
Identify in the field by looking for 'compositional banding' where light minerals (quartz/feldspar) and dark minerals (biotite/hornblende) are separated into distinct layers. Found globally in the roots of mountain ranges (e.g., Appalachians, Alps, Himalayas).
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