metamorphic
Granitic Gneiss
Gneiss (quartz-feldspar-mica complex)
Hardness: 6-7 Mohs; Color: gray, white, and black with iron staining; Luster: dull to vitreous; Structure: foliated with coarse grains; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 Mohs
- Color
- gray, white, and black with iron staining
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: gray, white, and black with iron staining; Luster: dull to vitreous; Structure: foliated with coarse grains; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous rock (granite) or sedimentary rocks. Subjected to intense heat and pressure, causing minerals to segregate into bands or lenses.
Uses & applications
Used in construction as crushed stone or decorative architectural stone. High durability makes it suitable for flooring and memorial markers.
Geological facts
Gneiss is some of the oldest rock on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term comes from an old German word meaning 'sparkling'.
Field identification & locations
Identify by looking for distinct mineral banding or 'gneissic foliation' and visible crystals of quartz and feldspar. Commonly found in the cores of mountain ranges and glacial till.
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