metamorphic
Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 (Mohs scale); Color: Banded appearance with alternating light (quartz/feldspar) and dark (biotite/amphibole) layers; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poor, typically breaks into irregular chunks; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded appearance with alternating light (quartz/feldspar) and dark (biotite/amphibole) layers; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poor, typically breaks into irregular chunks; 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). It is created under intense heat and pressure, often during mountain-building events (orogenies). It represents a more advanced metamorphic stage than schist or phyllite.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and concrete aggregate. High-quality specimens are polished for use as building facades, flooring, and kitchen countertops (often marketed commercially as 'granite').
Geological facts
Gneiss is among the oldest known rocks on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark,' because the rock can glint in the light due to its mineral content.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—thick bands of minerals that do not easily flake apart like schist. It is found in the deeply eroded cores of mountain ranges and ancient continental shields (e.g., the Appalachians, the Alps, and the Canadian Shield). Collectors look for specimens with sharply defined, high-contrast color bands.
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