metamorphic
Gneiss
Gneiss (quartz-feldspar-biotite gneiss)
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and tan with dark bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissose fabric); Cleavage: Poor, typically breaks along grain boundaries; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, and tan with dark bands
- Luster
- Dull to vitreous
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: Gray, pink, white, and tan with dark bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissose fabric); Cleavage: Poor, typically breaks along grain boundaries; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale). It is subjected to high temperatures and pressures, typically associated with mountain-building events (orogenies) often dating back hundreds of millions to billions of years.
Uses & applications
Used primarily as a construction material (crushed stone for road bases), dimension stone for building facades, paving stones, and sometimes for kitchen countertops and gravestones.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known crustal rock formations on Earth, estimated to be approximately 4.03 billion years old. The term comes from the Middle High German word 'gneist', meaning 'to spark'.
Field identification & locations
Identify in the field by looking for distinct 'zebra' banding where light minerals (quartz/feldspar) and dark minerals (biotite/hornblende) are separated into layers. It does not split into thin sheets as easily as schist. Commonly found in the cores of mountain ranges and ancient continental shields.
More like this
