metamorphic
Gneiss
Gneiss
Hardness: 6-7 (Mohs scale); Color: Typically alternating light and dark bands (gray, white, pink, black); Luster: Non-foliated to slightly vitreous; Structure: Gneissic banding (foliated); Cleavage: Usually absent in the rock as a whole; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Luster
- Non-foliated to slightly vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Typically alternating light and dark bands (gray, white, pink, black); Luster: Non-foliated to slightly vitreous; Structure: Gneissic banding (foliated); Cleavage: Usually absent in the rock as a whole; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) subjected to intense heat and pressure deep within the Earth's crust, often during mountain-building events (orogenies).
Uses & applications
Commonly used as a dimension stone for flooring, countertops, and grave markers; also used as crushed stone in road construction and landscaping.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dating back nearly 4.03 billion years. The word comes from a Middle High German verb meaning 'to spark', referring to the way the rock glitters.
Field identification & locations
Identified by its characteristic coarse-grained 'banding' where minerals have separated into distinct layers. Found worldwide in the core of mountain ranges and ancient continental shields. For collectors, look for high-contrast banding and mineral inclusions.
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