Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale; Color: banded gray, white, and black; Luster: matte to pearly; Crystal structure: Coarse-grained foliated texture (gneissose banding); Cleavage: poor, tends to break along bands.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under extreme heat and pressure, often during mountain-building events (orogeny).

Uses & Applications

Used in construction as crushed stone for road bases and concrete, or as dimension stone for architectural facings, flooring, and gravestones.

Geological Facts

Gneiss is one of the oldest rocks found on Earth; the Acasta Gneiss in Canada is dated to approximately 4 billion years old. The banding occurs due to the segregation of light minerals like quartz and feldspar from dark minerals like biotite and amphibole.

Field Identification & Locations

Identified in the field by its distinct alternating layers of dark and light minerals (foliation) that are thicker than the fine layers in schist. Commonly found in the crystalline cores of mountain ranges and stable continental shields.

Identified on: 5/1/2026

Mode: Standard