Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and black; Luster: Dull to vitreous; Crystal structure: Coarse-grained with distinct foliation (gneissose banding); Cleavage: Poor, though it may split along bands.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under intense heat and pressure deep within the Earth's crust.

Uses & Applications

Used primarily as crushed stone for road construction and concrete aggregate, or as decorative building stone for countertops, flooring, and gravestones.

Geological Facts

Gneiss contains some of the oldest known rock on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. Its name comes from a German word meaning 'sparkling' or 'to spark'.

Field Identification & Locations

Identify in the field by looking for 'gneissose banding', where light-colored minerals (quartz/feldspar) and dark-colored minerals (biotite/amphibole) are separated into distinct layers. Found worldwide in ancient crystalline shields and mountain cores.

Identified on: 4/22/2026

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