Gneiss

Physical Properties

Hardness: 6-7 (Mohs); Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding; Gravity: 2.6-2.9.

Formation & Geological History

Formed by high-grade regional metamorphism of igneous or sedimentary rocks (like granite or shale) under extreme pressure and temperature. It is often found in the cores of mountain ranges and ancient shield areas.

Uses & Applications

Used primarily as crushed stone for road construction and concrete aggregate. High-quality aesthetic pieces are used as dimension stone for countertops, flooring, and decorative building facades.

Geological Facts

Gneiss is one of the oldest rock types found on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding is caused by 'gneissic foliation' where minerals segregate into layers.

Field Identification & Locations

Identify by coarse-grained crystals and distinct 'salt and pepper' banding (foliation) where layers of light and dark minerals are at least 1mm thick. Often found in deeply eroded mountain belts.

Identified on: 4/20/2026

Mode: Standard