metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Dull to vitreous with sparkly micaceous flakes; Crystal Structure: Coarse-grained foliated (gneissic banding); Cleavage: Structural cleavage along mineral bands.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous with sparkly micaceous flakes
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: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Dull to vitreous with sparkly micaceous flakes; Crystal Structure: Coarse-grained foliated (gneissic banding); Cleavage: Structural cleavage along mineral bands.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under intense heat and pressure. This specimen shows a rounded shape indicative of water erosion in a river or glacial environment.
Uses & applications
Used primarily in construction for crushed stone, road base, and as dimension stone for countertops, flooring, and gravestones. It is also an important indicator rock for geological mapping of tectonic history.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dated at 4.03 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' or 'glimmer', referencing its sparkly mineral inclusions.
Field identification & locations
Identify in the field by looking for 'gneissic banding', where minerals are segregated into distinct light and dark layers. Common in continental shields and mountain roots. For collectors, look for distinct banding patterns and lack of easy splitting compared to schist.
More like this
