Gneiss

Physical Properties

Displays distinct 'gneissic banding' or foliation. Hardness: 6-7 on Mohs scale. Colors: Alternating light (quartz/feldspar) and dark (biotite/hornblende) layers. Luster: Dull to vitreous. Texture: Coarse-grained and crystalline.

Formation & Geological History

Formed through high-grade regional metamorphism of igneous (orthogneiss) or sedimentary (paragneiss) rocks. It occurs at high temperatures and pressures deep within the Earth's crust, often during mountain-building events (orogenies).

Uses & Applications

Used primarily as a crushed stone for road construction and concrete aggregate. High-quality slabs can be used as dimension stone for flooring, countertops, and building facades. It is also a common specimen for educational geological collections.

Geological Facts

Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, which dates back over 4 billion years. The striping is not layering in the traditional sedimentary sense but is caused by minerals segregating under intense pressure.

Field Identification & Locations

Identify in the field by looking for alternating bands of different mineral compositions (banding) that are often folded or wavy. It is widespread in the 'basement' of continental crust and often found in mountain ranges and glacial deposits.

Identified on: 4/20/2026

Mode: Standard