metamorphic
Banded Gneiss
Gneiss (Metamorphic rock composed of feldspar, quartz, and mica)
Hardness: 6-7 on Mohs scale; Color: Alternating light (gray/white) and dark (black) bands; Luster: Dull to pearly; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Poor, typically breaks along planes between bands.
- Hardness
- 6-7 on Mohs scale
- Color
- Alternating light (gray/white) and dark (black) bands
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating light (gray/white) and dark (black) bands; Luster: Dull to pearly; Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Poor, typically breaks along planes between bands.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under intense heat and pressure. Gneiss is often very old, frequently dating back to the Precambrian era.
Uses & applications
Used primarily as a crushed stone in road construction and as a dimension stone for flooring, cemetery monuments, and building facing. Highly polished specimens are used for countertops.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada estimated to be about 4 billion years old. The term 'Gneiss' comes from an old German mining term meaning 'sparkling'.
Field identification & locations
Identify in the field by looking for 'salt and pepper' appearances and the distinct segregation of light-colored minerals (quartz/feldspar) and dark minerals (biotite/hornblende) into bands. Commonly found in the crystalline cores of mountain ranges.
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