Banded Gneiss

Physical Properties

Hardness: 6-7 (Mohs); Color: Alternating bands of dark brown/black and lighter reddish-brown (likely orthoclase or garnet-rich); Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic fabric); Specific Gravity: 2.6-2.9

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense pressure and temperature, causing minerals to segregate into bands. Often Precambrian in age, ranging from 500 million to 4 billion years old.

Uses & Applications

Used primarily as crushed stone for road construction and concrete aggregate; high-quality specimens are used as dimension stone for countertops, flooring, and 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. The banding is caused by 'metamorphic differentiation' where minerals recrystallize into layers without melting.

Field Identification & Locations

Identify by looking for distinct mineral banding (foliation) that is thicker and coarser than schist. It is commonly found in the roots of mountain ranges and ancient continental shields. Collectors look for sharp contrast between layers and presence of accessory minerals like garnets.

Identified on: 4/28/2026

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