Gneiss (Metamorphic Rock)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Banded pink, brown, and black; Luster: Variable (dull to vitreous when wet); Structure: Foliated with distinct bands; Cleavage: Poor; Specific Gravity: 2.6-3.0.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure. This specimen shows water-worn smoothing, typical of river or glacial transport.

Uses & Applications

Used in construction as crushed stone, dimension stone for flooring and countertops, and as ornamental landscaping stone.

Geological Facts

Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is over 4 billion years old. The banding is caused by the separation of minerals like quartz/feldspar (light) and biotite/amphibole (dark).

Field Identification & Locations

Identify in the field by looking for 'gneissose banding'—distinct light and dark layers. Unlike schist, it does not split easily along these planes. This specimen appears wet or polished, highlighting its internal mineral grains.

Identified on: 4/28/2026

Mode: Standard