metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Banded grey, black, pink, and white; Luster: Variable, often pearly or sub-vitreous; Crystal structure: Crystalline with visible minerals; Cleavage: Poor, but exhibits planar foliation; Specific gravity: 2.6 - 2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, black, pink, and white
- Luster
- Variable, often pearly or sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, black, pink, and white; Luster: Variable, often pearly or sub-vitreous; Crystal structure: Crystalline with visible minerals; Cleavage: Poor, but exhibits planar foliation; Specific gravity: 2.6 - 2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous (granite) or sedimentary (shale/mudstone) rocks under intense heat and pressure. Most specimens are Precambrian in age, often billions of years old.
Uses & applications
Commonly used as a decorative building stone, crushed stone for road construction, paving stones, and in landscaping. Polished samples are used for countertops and monuments.
Geological facts
Gneiss contains some of the oldest known rock on Earth, such as the Acasta Gneiss in Canada, dated to approximately 4.03 billion years old. The term 'gneiss' comes from an Old High German word meaning 'sparkling' or 'to glisten.'
Field identification & locations
Identifiable by its distinct 'gneissose banding'—alternating layers of light-colored felsic minerals (quartz/feldspar) and dark-colored mafic minerals (biotite/hornblende). Found in the cores of mountain ranges and ancient continental shields. For collectors, look for tight folds or 'micro-folding' within the bands.
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