Gneiss

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and black; Luster: Dull to vitreous; Crystal Structure: Coarse-grained foliated texture with distinct mineral bands; Specific Gravity: 2.6-2.9.

Formation & Geological History

Formed through high-grade regional metamorphism of igneous or sedimentary rocks at deep crustal levels under high pressure and temperature. Ages vary greatly, often part of ancient Precambrian shields.

Uses & Applications

Used primarily as a crushed stone for road construction and railroad ballast; higher quality specimens are used for architectural facing, countertops, and curbstone.

Geological Facts

Gneiss includes some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old. The banding is called 'gneissose banding'.

Field Identification & Locations

Identify in the field by looking for 'salt and pepper' appearance and distinct alternating dark and light mineral layers (foliation) that do not split easily like schist. Common in mountain ranges and glacial deposits.

Identified on: 4/26/2026

Mode: Standard