Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Gray with streaks of white, black, and sometimes yellow/brown minerals; Luster: Dull to vitreous; Crystal structure: Coarse-grained with distinct foliation or 'gneissic banding'; Cleavage: Poor, typically breaks along grain boundaries.

Formation & Geological History

Formed from the high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under extreme heat and pressure. It is often Archean to Proterozoic in age, found in the roots of mountain ranges.

Uses & Applications

Used primarily in construction as crushed stone for road bases, and when polished, as high-end dimensional stone for countertops, flooring, and architectural facades.

Geological Facts

Gneiss is one of the oldest known rocks on Earth. The Acasta Gneiss in Canada is approximately 4 billion years old, making it one of the oldest crustal rocks ever discovered.

Field Identification & Locations

Identify in the field by looking for distinct light and dark mineral bands (segregation of minerals) that are thicker and more irregular than the layers in schist. It is commonly found in the crystalline cores of mountain belts and shield areas like the Canadian Shield.

Identified on: 4/16/2026

Mode: Standard