metamorphic
Gneiss
Gneiss (Metamorphic rock dominated by Quartz, Feldspar, and various dark minerals)
Hardness: 6-7 on Mohs scale; Color: Grey and white banded with possible dark specks; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Grey and white banded with possible dark specks
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grey and white banded with possible dark specks; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures and pressures. Most gneiss is deeply buried and formed during mountain-building events (Orogeny).
Uses & applications
Used primarily as crushed stone for road construction and railroad ballast, or as a dimension stone for architectural facades, countertops, and flooring when it displays attractive banding.
Geological facts
Gneiss is one of the oldest known rocks on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term 'gneiss' comes from an Old German word meaning 'sparkling' or 'to glow'.
Field identification & locations
Identified in the field by its distinct 'gneissic banding' where minerals are separated into light (felsic) and dark (mafic) visible layers. Common in the cores of mountain ranges and ancient continental shields.
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