Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Banded pink, gray, and black. Luster: Dull to pearly. Crystal structure: Coarse-grained with distinct foliation (gneissic banding). Specific gravity: 2.6-2.9.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures and pressures. Often Precambrian in age (billions of years old).

Uses & Applications

Used in construction as crushed stone or dimension stone for countertops, flooring, and decorative facades. Also popular in landscaping.

Geological Facts

Gneiss contains some of the oldest known rock crust on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old.

Field Identification & Locations

Identify by its 'gneissic banding' where light and dark minerals are separated into distinct layers. Commonly found in the roots of mountain ranges and glacial deposits.

Identified on: 5/3/2026

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