metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and black with occasional pink or red tints from garnet or feldspar; Luster: vitreous to dull; Crystal structure: crystalline but with a foliated, banded texture; Cleavage: poor; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and black with occasional pink or red tints from garnet or feldspar; Luster: vitreous to dull; Crystal structure: crystalline but with a foliated, banded texture; 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 within the Earth's crust.
Uses & applications
Commonly used in construction as crushed stone for road bases and concrete, or as dimension stone for architectural cladding and kitchen countertops when polished.
Geological facts
The Acasta Gneiss in Canada is considered one of the oldest known intact crustal fragments on Earth, dating back approximately 4.03 billion years.
Field identification & locations
Identify in the field by looking for distinct alternating layers of light-colored minerals (quartz/feldspar) and dark-colored minerals (biotite/hornblende). It is extremely common in mountain ranges and continental shields globally.
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