Gneiss

Physical Properties

Hardness: 6-7 (Mohs). Color: Gray, pink, white, and black banded patterns. Luster: Dull to vitreous. Crystal structure: Coarse-grained with visible regional foliation (gneissic banding). Specific gravity: 2.6-2.9.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under extreme heat and pressure. It often dates back to Precambrian or Paleozoic eras.

Uses & Applications

Commonly used in construction as crushed stone for road bases and concrete, or as dimension stone for building facades, floor tiles, and grave markers.

Geological Facts

Gneiss is one of the oldest types of rock found on Earth, with the Acasta Gneiss in Canada estimated to be approximately 4 billion years old.

Field Identification & Locations

Identify in the field by looking for distinct alternating light and dark bands (gneissic banding) and large, visible mineral grains. It is extremely common in the roots of mountain ranges and shield areas worldwide.

Identified on: 4/20/2026

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