metamorphic
Banded Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Alternating bands of dark (biotite/amphibole) and light (quartz/feldspar); Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating bands of dark (biotite/amphibole) and light (quartz/feldspar); Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing sedimentary or igneous rocks under intense heat and pressure. It often dates back to the Precambrian era, representing some of the oldest crust on Earth.
Uses & applications
Used in construction as crushed stone, dimension stone for countertops and flooring, and as landscaping material. Smaller, attractive specimens are often collected as river stones or palm stones.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens in the Acasta Gneiss complex dating back over 4 billion years. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' or 'to glisten'.
Field identification & locations
Identified by 'gneissose banding' where minerals are separated into distinct layers. Frequently found in the roots of mountain ranges or in glacial tills. Collectors look for high contrast between bands and smooth water-worn textures.
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