metamorphic
Gneiss
Feldspar–quartz–mica gneiss, locally amphibole-bearing
AI-generated identification · may be incorrectMedium- to coarse-grained, with pale quartz/feldspar and dark biotite or amphibole arranged in irregular compositional bands. Hardness is about 6–7, luster vitreous to pearly, and cleavage is weak overall but strong in individual feldspar or mica grains; approximate specific gravity is 2.6–2.9.
Identified More metamorphic →
Explore Gneiss in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Medium- to coarse-grained, with pale quartz/feldspar and dark biotite or amphibole arranged in irregular compositional bands. Hardness is about 6–7, luster vitreous to pearly, and cleavage is weak overall but strong in individual feldspar or mica grains; approximate specific gravity is 2.6–2.9.
Formation & geological history
Formed when preexisting granite or other rocks underwent intense high-grade metamorphism, producing mineral segregation and banding. Such rocks commonly formed during Precambrian to Paleozoic mountain-building events and were later rounded by erosion.
Uses & applications
Used as crushed stone, road aggregate, building stone, landscaping rock, and occasionally decorative facing or countertops; this rounded specimen is mainly a collecting or educational sample.
Geological facts
Its banding reflects metamorphic recrystallization rather than sedimentary layering. The dark minerals are likely biotite mica and/or amphibole, while the light areas are mostly quartz and feldspar.
Field identification & locations
Identify it by coarse interlocking grains and light–dark banding, unlike granite’s more random texture; a hand lens and fresh surface help confirm the minerals. Common in ancient continental shields and exposed mountain belts.
More like this
