Gneiss (with Quartz and Biotite Mica)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Gray, white, and black banded/mottled appearance; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: Generally poor, though mica flakes show perfect basal cleavage; Specific gravity: 2.6-2.9.

Formation & Geological History

Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/mudstone) protoliths. It undergoes intense heat and pressure deep within the Earth's crust, leading to the segregation of minerals into bands.

Uses & Applications

Used primarily as a crushed stone for road construction, railroad ballast, and concrete aggregate. High-quality specimens with attractive banding are used for building stone, countertops, and decorative landscaping.

Geological Facts

Gneiss contains some of the oldest rock formations on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. The term comes from an Old High German word meaning 'spark' or 'to glisten'.

Field Identification & Locations

Identify in the field by looking for 'gneissose banding'—distinct light and dark layers of minerals. It is commonly found in the cores of mountain ranges and ancient continental shields. Look for visible flakes of mica and glassy quartz grains.

Identified on: 4/22/2026

Mode: Standard