metamorphic
Gneiss (with iron staining)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Grey, white, and black banded with orange-brown iron oxidation; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Cleavage: Poorly defined; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grey, white, and black banded with orange-brown iron oxidation; Luster: Dull to vitreous; Structure: Foliated (gneissic banding); Cleavage: Poorly defined; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure, typically deep within the earth's crust during mountain-building events (orogeny).
Uses & applications
Used primarily as crushed stone for road construction, concrete aggregate, and as decorative dimension stone for countertops, flooring, and curbing.
Geological facts
Gneiss contains some of the oldest known rock formations on Earth, such as the Acasta Gneiss in Canada which is approximately 4 billion years old. The term comes from the Middle High German verb 'gneist' meaning to spark.
Field identification & locations
Identify in the field by its distinct alternating bands of light-colored (quartz/feldspar) and dark-colored (biotite/amphibole) minerals. Look for 'augen' or eye-shaped mineral grains. Common in continental shields and mountain ranges.
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