Gneiss (Granitic Gneiss)

Physical Properties

Hardness 6-7 (Mohs); Color: alternating light (pinkish-tan feldspar/quartz) and dark (black biotite/hornblende) bands; Luster: Vitreous to dull; Crystal structure: Course-grained, foliated; Cleavage: Usually none at rock level.

Formation & Geological History

Formed through high-grade regional metamorphism of igneous (granite) or sedimentary rocks under extreme pressure and temperature. It is typical of Precambrian cratons and ancient mountain belts.

Uses & Applications

Used as crushed stone in construction, dimension stone for flooring and counter tops, and as decorative stone in landscaping.

Geological Facts

Gneiss contains some of the oldest rock formations on Earth, such as the Acasta Gneiss which is over 4 billion years old. The term comes from an Old German word meaning 'sparkling'.

Field Identification & Locations

Identified in the field by its distinct 'gneissose banding'—parallel layers of differing mineral colors. It is common in mountain ranges and glacial deposits across North America (Canadian Shield) and Europe.

Identified on: 4/22/2026

Mode: Standard