metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, or brown; Luster: vitreous to pearly; Structure: foliated with distinct banding (gneissose), coarse-grained; Cleavage: poor; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- typically banded gray, white, or brown
- Luster
- vitreous to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, or brown; Luster: vitreous to pearly; Structure: foliated with distinct banding (gneissose), coarse-grained; Cleavage: poor; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under intense heat and pressure deep in the Earth's crust.
Uses & applications
Used in construction as crushed stone for road bases and building foundations. Also used as dimension stone for architectural facades, countertops, and flooring when polished.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known rock formations on Earth, dating back approximately 4.03 billion years. Its characteristic bands are caused by the separation of mineral layers during intense deformation.
Field identification & locations
Identify in the field by looking for 'gneissose banding' - alternating layers of light-colored minerals (quartz/feldspar) and dark-colored minerals (biotite/amphibole). It is found globally in the cores of mountain ranges and ancient continental shields.
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