metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 Mohs; Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poor, though it may split along dark bands; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 Mohs
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poor, though it may split along dark bands; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism under extreme pressure and temperature (up to 700°C). It usually originates from granite (orthogneiss) or sedimentary rocks like shale/sandstone (paragneiss). These rocks are often billions of years old, forming the 'basement' of many continental plates.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and as dimension stone for architectural facings, flooring, and decorative landscaping. High-quality specimens with unique patterns may be used as ornamental stones.
Geological facts
Gneiss is one of 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.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—distinctive, thick, parallel layers of alternating mineral colors that don't split as easily as slate or schist. To find it, look in the roots of ancient mountain ranges or deep glacial till. Collectors value pieces with 'folded' bands that show tectonic movement.
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