Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Gray, black, pink, or white; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.

Formation & Geological History

Formed through high-grade regional metamorphism of igneous or sedimentary rocks (such as granite or shale). It is subjected to intense heat and pressure, causing minerals to segregate into bands. Often found in ancient shields or core mountain ranges.

Uses & Applications

Used primarily as crushed stone for road construction and aggregate. High-quality specimens are used as building stone, facing for buildings, curbing, and decorative landscaping.

Geological Facts

Gneiss contains some of the oldest rock formations on Earth, such as the Acasta Gneiss in Canada which is roughly 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' or 'to glisten.'

Field Identification & Locations

Identify in the field by looking for alternating light-colored (quartz/feldspar) and dark-colored (biotite/amphibole) mineral bands. Unlike schist, it does not split easily along planes. Commonly found in metamorphic terrains across the globe.

Identified on: 4/14/2026

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