metamorphic
Gneiss (specifically a Granitic Gneiss specimen)
Gneiss (high-grade metamorphic rock primarily composed of Quartz, Feldspar, and Biotite/Amphibole)
Hardness: 6-7 on Mohs scale; Color: Pinkish-orange, tan, and dark grey/black bands; Luster: Vitreous to dull; Structure: Gneissic banding (foliated); Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Pinkish-orange, tan, and dark grey/black bands
- Luster
- Vitreous to dull
Identified More metamorphic →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 on Mohs scale; Color: Pinkish-orange, tan, and dark grey/black bands; Luster: Vitreous to dull; Structure: Gneissic banding (foliated); Specific Gravity: 2.6-2.9.
Formation & geological history
Formed via high-grade regional metamorphism of granite or sedimentary protoliths under intense heat and pressure. Commonly dates back to the Precambrian or Paleozoic eras within deep crustal environments.
Uses & applications
Used primarily as crushed stone for road construction, landscaping boulders, and occasionally as architectural facing or dimension stone (granite-like countertops).
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada which is over 4 billion years old. The 'eyes' or lens-shaped minerals sometimes found in it are called porphyroblasts.
Field identification & locations
Identify by looking for distinct 'segregation' bands where light minerals (quartz/feldspar) and dark minerals (biotite/hornblende) separate. This smooth, water-worn cobble shape suggests it was found in a riverbed or glacial deposit. Common in mountainous regions like the Appalachians or Canadian Shield.
More like this
