metamorphic
Gneiss
Gneiss (foliated metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Grayish-green to dark gray with faint bandings and flecks of shimmering mica/quartz; Luster: Pearly to vitreous on flecks, earthy overall; Texture: Medium- to coarse-grained foliated texture with distinct aligned mineral grains.
- Hardness
- 6-7 on Mohs scale
- Luster
- Pearly to vitreous on flecks, earthy overall
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grayish-green to dark gray with faint bandings and flecks of shimmering mica/quartz; Luster: Pearly to vitreous on flecks, earthy overall; Texture: Medium- to coarse-grained foliated texture with distinct aligned mineral grains.
Formation & geological history
Formed by high-grade regional metamorphism of igneous (like granite) or sedimentary rocks under intense heat and pressure deep within Earth's crust. Commonly found in ancient shield regions and mountain belts.
Uses & applications
Used as crushed stone in construction, road base, landscaping river rock, and occasionally as decorative building stone or paving cobbles.
Geological facts
Gneiss is one of the oldest rock types on Earth, with some specimens like the Acasta Gneiss in Canada dating back over 4 billion years. The texture seen here is well-rounded due to natural stream or wave erosion.
Field identification & locations
Identified by alternating light and dark mineral bands or strong foliation with visible shimmering mica flakes. Highly abundant worldwide in glaciated terrains, riverbeds, and mountainous regions. Value is strictly educational or decorative.
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