metamorphic
Gneiss (Hornblende/Biotite Gneiss)
Foliated Metamorphic Rock (Quartz-Feldspar-Amphibole/Mica matrix)
Hardness: 6-7 on Mohs scale; Color: Alternating dark black/dark grey bands and lighter greyish-tan bands; Luster: Sub-vitreous to dull; Texture: Medium to coarse-grained with distinct foliation (gneissic banding); Cleavage: Uneven to sub-conchoidal fracture.
- Hardness
- 6-7 on Mohs scale
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating dark black/dark grey bands and lighter greyish-tan bands; Luster: Sub-vitreous to dull; Texture: Medium to coarse-grained with distinct foliation (gneissic banding); Cleavage: Uneven to sub-conchoidal fracture.
Formation & geological history
Formed through high-grade regional metamorphism of igneous rocks (such as granite or diorite) or sedimentary rocks (such as shale or graywacke) under extreme heat and pressure deeply within the Earth's crust.
Uses & applications
Used widely as a crushed stone in construction, road base, and as decorative stone, facing stone, or landscaping gravel. Polished slabs are sometimes used for countertops.
Geological facts
The distinct light and dark bands in gneiss are caused by mineral segregation under directional stress, separating mafic minerals (like amphibole/hornblende and biotite) from felsic minerals (like quartz and feldspar). Some of the oldest rocks on Earth, like the Acasta Gneiss in Canada (~4.0 billion years old), are gneiss.
Field identification & locations
Identified in the field by its characteristic alternating dark and light mineral bands (gneissic banding) and granular texture without easy cleavage splitting. Commonly found in shield regions, eroded mountain roots, and glacial deposits around the world.
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