Gneiss (Metamorphic Rock)

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Gray, pink, white, and black banded patterns. Luster: Vitreous to dull. Structure: Foliated with distinct bands (gneissose banding). Cleavage: Generally poor, breaks along blocky joints.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale/mudstone). Subjected to high temperatures and pressures deep within the Earth's crust, typically during mountain-building events (orogenies).

Uses & Applications

Used as a decorative building stone, crushed stone for road construction and railroad ballast, and occasionally for landscaping or as a polishing material in rock tumblers.

Geological Facts

Gneiss is one of the oldest known rock types on Earth, with the Acasta Gneiss in Canada being approximately 4 billion years old. The term 'gneiss' comes from an old German word meaning 'sparkling' or 'to glow'.

Field Identification & Locations

Identify in the field by its characteristic alternating bands of light-colored (felsic) minerals like quartz/feldspar and dark-colored (mafic) minerals like biotite/hornblende. Commonly found in the crystalline cores of mountain ranges.

Identified on: 4/28/2026

Mode: Standard