metamorphic
Gneiss
High-grade regional metamorphic Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating light (quartz, feldspar) and dark (biotite, amphibole) layers; Luster: vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: poor, but breaks along foliation planes. Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- 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: typically banded with alternating light (quartz, feldspar) and dark (biotite, amphibole) layers; Luster: vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: poor, but breaks along foliation planes. Specific gravity: 2.6-2.9.
Formation & geological history
Formed by high-pressure, high-temperature regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale). Most gneiss is Precambrian in age, often billions of years old, forming the 'basement' of continents.
Uses & applications
Extensively used as a dimension stone for flooring, wall facing, and building facades. In crushed form, it is used for road construction, railroad ballast, and concrete aggregate. Select specimens are used as decorative garden stones.
Geological facts
Gneiss is one of the oldest known rocks on Earth. The Acasta Gneiss in Canada is approximately 4.03 billion years old, nearly as old as the planet itself. It represents a very high grade of metamorphism, just before the rock would begin to melt into magma.
Field identification & locations
Identify in the field by searching for 'gneissic banding'—distinctive color stripes or layers that are often wavy or contorted. Unlike schist, it does not split easily into thin sheets. Found in the cores of mountain ranges and eroded shield areas.
More like this
