metamorphic

Banded Gneiss

Gneiss (High-grade Regional Metamorphic Rock)

AI-generated identification · may be incorrect

Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and tan; Luster: Dull to pearly; Crystal structure: Coarse-grained foliated texture (gneissic banding); Cleavage: Poor, but splits alongside foliation planes; Specific gravity: 2.6–2.9.

Hardness: 6-7 (Mohs scale)
Color: Banded grey, white, and tan
Luster: Dull to pearly

Identified March 29, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths). Produced under extreme pressure and temperature (over 600°C) which causes minerals to segregate into distinct bands. Often dates back to the Precambrian or Paleozoic eras.

Uses & applications

Used primarily as a crushed stone for road construction, railroad ballast, and concrete aggregate. High-quality slabs are used for architectural facings, flooring, and decorative garden stones.

Geological facts

Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. Because of its extreme formation conditions, it rarely contains fossils.

Field identification & locations

Identify in the field by looking for distinct alternating layers (bands) of light (quartz/feldspar) and dark (biotite/hornblende) minerals. Commonly found in the crystalline cores of mountain ranges and ancient continental shields. Collectors should look for sharp color contrast and large mineral inclusions.