metamorphic

Gneiss

AI-generated identification · may be incorrect

Hardness: 6-7 (Mohs); Color: Banded grey, white, and black with visible mica; Luster: Dull to vitreous; Crystal structure: Coarse-grained foliated; Cleavage: Poor, but splits along bands; Specific gravity: 2.6-2.9

Hardness: 6-7 (Mohs)
Color: Banded grey, white, and black with visible mica
Luster: Dull to vitreous

Identified July 3, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure, typically found in ancient mountain belts and continental shields.

Uses & applications

Used in construction as crushed stone for road bases and as a decorative building stone (dimension stone) for flooring, gravestones, and countertops.

Geological facts

The oldest known rocks on Earth are the Acasta Gneiss in Canada, which are over 4 billion years old. The distinctive banding is called 'gneissic banding' and results from the segregation of different mineral types.

Field identification & locations

Identify in the field by looking for distinct alternating layers of light-colored (quartz/feldspar) and dark-colored (mafic) minerals. Common in the Appalachian Mountains and the Canadian Shield.