Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Grey and white banded; Luster: Variable, often dull with sparkling mineral grains; Structure: Foliated with distinct bands of minerals; Cleavage: None, breaks irregularly.

Formation & Geological History

Formed through high-grade regional metamorphism of igneous or sedimentary rocks under intense heat and pressure, typically during mountain-building events.

Uses & Applications

Used as building stone, decorative landscaping, and occasionally as crushed stone for aggregate in road construction.

Geological Facts

The Gneissic banding is a result of mineral segregation where light and dark minerals separate into layers. The Acasta Gneiss in Canada is one of the oldest known rock units on Earth, at roughly 4 billion years old.

Field Identification & Locations

Identify by visible color-coordinated stripes (foliation) and lack of distinct cleavage. Often found in continental shields and core of mountain ranges. Collectors look for unique banding patterns.

Identified on: 5/3/2026

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