Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banding; Luster: Dull to vitreous; Structure: Foliated with distinct mineral bands (gneissose banding); Specific Gravity: 2.6-2.9.

Formation & Geological History

Formed by high-grade regional metamorphism of pre-existing sedimentary rocks (like shale) or igneous rocks (like granite). It occurs at high temperatures and pressures deep within the Earth's crust, often during mountain-building events (orogenies).

Uses & Applications

Used as a crushed stone in road construction and concrete. Polished slabs are used for flooring, countertops, and ornamental building stone.

Geological Facts

Gneiss is among the oldest known rocks on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The term 'gneiss' comes from a German word meaning 'sparkling' or 'bright'.

Field Identification & Locations

Identify in the field by looking for 'salt and pepper' appearance with distinct parallel bands or layers of light and dark minerals. It will not split into thin sheets as easily as schist. Commonly found in the cores of mountain ranges and ancient continental shields.

Identified on: 4/28/2026

Mode: Standard