metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs); Color: banding of grey, white, and tan; Luster: earthy to vitreous; Structure: foliated with distinct banding (gneissic foliation); Cleavage: poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs)
- Color
- banding of grey, white, and tan
- Luster
- earthy to vitreous
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Physical properties
Hardness: 6-7 (Mohs); Color: banding of grey, white, and tan; Luster: earthy to vitreous; Structure: foliated with distinct banding (gneissic foliation); Cleavage: poor; Specific Gravity: 2.6-2.9
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure, causing segregation of minerals into alternating layers.
Uses & applications
Used as crushed stone for road construction, as building stone, and occasionally for countertops and floor tiles.
Geological facts
Gneiss is some of the oldest rock found on Earth, with some specimens in Canada dating back over 4 billion years. The banding is caused by the separation of light minerals like quartz and feldspar from dark minerals like biotite and hornblende.
Field identification & locations
Identified in the field by its distinct, often wavy or distorted banding known as 'gneissic foliation.' Common in the core of mountain ranges and ancient continental shields.
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