metamorphic
Gneiss
High-grade Metamorphic Gneiss
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and black with possible pink or tan streaks; Luster: Vitreous to dull; Crystal structure: Foliated (banded); Cleavage: Poor; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and black with possible pink or tan streaks; Luster: Vitreous to dull; Crystal structure: Foliated (banded); Cleavage: Poor; Specific gravity: 2.6-2.9
Formation & geological history
Formed via high-grade regional metamorphism of igneous or sedimentary precursor rocks (protoliths) under intense heat and pressure, typically deep within the Earth's crust or during mountain-building events.
Uses & applications
Used as a crushed stone in construction (road base), dimension stone for architectural facings, countertops (where it is often sold commercially as 'granite'), and as a decorative landscaping rock.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada being dated to approximately 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' due to its glimmering minerals.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—distinct light and dark layers of minerals like quartz, feldspar, and mica. It is common in the roots of mountain ranges and ancient continental shields.
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