metamorphic
Banded Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
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
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Physical properties
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.
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.
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