metamorphic
Gneiss (specifically a Banded Gneiss cobble)
Gneiss (composed mainly of Quartz, Feldspar, and Biotite/Amphibole)
Hardness: 6-7 on Mohs scale; Color: Pink, white, and grey with black bands; Luster: Dull to vitreous; Crystal Structure: Coarse-grained, non-porous; Cleavage: Poor (tends to break across bands); Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Pink, white, and grey with black bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pink, white, and grey with black bands; Luster: Dull to vitreous; Crystal Structure: Coarse-grained, non-porous; Cleavage: Poor (tends to break across bands); Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary protoliths under intense heat and pressure. This specific specimen is a 'glacial erratic' or river cobble, rounded by water or ice transport. Most gneiss dates to the Precambrian or Paleozoic eras.
Uses & applications
Used in construction as crushed stone or 'dimension stone' for flooring, gravestones, and countertops. Often used as decorative landscaping rock.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with the Acasta Gneiss in Canada being over 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning to spark or glisten.
Field identification & locations
Identify by 'gneissose banding' (distinct alternating layers of light and dark minerals). Found globally in continental shields and mountain belts. Collectors should look for sharp contrasts in bands and evidence of folding or 'ptygmatic' veins.
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