Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and dark minerals; Luster: dull to vitreous; Crystal structure: foliated/gneissose banding; Cleavage: poor; Specific gravity: 2.6-2.9.

Formation & Geological History

Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) rocks under extreme pressure and temperature. Foundations of continents, often billions of years old.

Uses & Applications

Used as a dimension stone for flooring, cemetery markers, facing on buildings, and as crushed stone for road construction and landscaping.

Geological Facts

The Acasta Gneiss in Canada is one of the oldest known intact crustal rock fragments on Earth, dated to approximately 4.03 billion years ago.

Field Identification & Locations

Identify by distinct 'gneissose banding' – alternating layers of light-colored (quartz/feldspar) and dark-colored (biotite/amphibole) minerals. Found in mountainous regions and glacial tills.

Identified on: 5/3/2026

Mode: Standard