metamorphic
Gneiss (Granitic Gneiss)
Gneiss (composed primarily of Quartz, Feldspar, and Mica/Amphibole)
Hardness 6-7 (Mohs); Color: alternating light (pinkish-tan feldspar/quartz) and dark (black biotite/hornblende) bands; Luster: Vitreous to dull; Crystal structure: Course-grained, foliated; Cleavage: Usually none at rock level.
- Luster
- Vitreous to dull
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Physical properties
Hardness 6-7 (Mohs); Color: alternating light (pinkish-tan feldspar/quartz) and dark (black biotite/hornblende) bands; Luster: Vitreous to dull; Crystal structure: Course-grained, foliated; Cleavage: Usually none at rock level.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary rocks under extreme pressure and temperature. It is typical of Precambrian cratons and ancient mountain belts.
Uses & applications
Used as crushed stone in construction, dimension stone for flooring and counter tops, and as decorative stone in landscaping.
Geological facts
Gneiss contains some of the oldest rock formations on Earth, such as the Acasta Gneiss which is over 4 billion years old. The term comes from an Old German word meaning 'sparkling'.
Field identification & locations
Identified in the field by its distinct 'gneissose banding'—parallel layers of differing mineral colors. It is common in mountain ranges and glacial deposits across North America (Canadian Shield) and Europe.
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