Gneiss (with Quartzite and Biotite)

Physical Properties

Hardness: 6-7 Mohs; Color: Gray, tan, and reddish-brown layers; Luster: Dull to vitreous; Crystal structure: Phaneritic (medium to coarse grains); Cleavage: Indistinct but exhibits foliation banding.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme heat and pressure. This specimen shows 'gneissic banding' and weathering stains of iron oxides.

Uses & Applications

Used primarily as crushed stone for road construction, railroad ballast, and building stone. Highly polished slabs are used for countertops and architectural facing.

Geological Facts

Gneiss can contain some of the oldest rocks on Earth, such as the Acasta Gneiss which is over 4 billion years old. The banding occurs because minerals segregate into layers during intense pressure.

Field Identification & Locations

Identify in the field by looking for distinct light and dark mineral bands (foliation) and a crystalline texture. It is a very hard rock that typically will not scratch easily with a steel knife.

Identified on: 4/15/2026

Mode: Standard