Gneiss (Granitic Gneiss)

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Banded gray, white, and pink/salmon; Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: None, but splits along bands.

Formation & Geological History

Formed by high-grade regional metamorphism of granite or sedimentary rocks at high temperatures and pressures deep within the Earth's crust, typically during mountain-building events (orogenies).

Uses & Applications

Used in construction as crushed stone for road bases, as dimension stone for architectural facing, flooring, and occasionally for graveyard monuments or decorative landscaping.

Geological Facts

Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dated to approximately 4.03 billion years old. The pink bands in this specimen are likely composed of potassium feldspar (orthoclase).

Field Identification & Locations

Identify by looking for 'segregated bands' of different minerals (light vs dark) that are thicker than those in schist. Found in the roots of mountain ranges and glacial till. This specific specimen is a glacial erratic, rounded by water or ice transport.

Identified on: 4/24/2026

Mode: Standard