metamorphic
Gneiss (specifically a water-worn pebble)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, pink, and black; Luster: Dull to vitreous; Crystal Structure: Foliated (gneissose banding); Cleavage: Poor, tends to break along bands; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, white, pink, and black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, pink, and black; Luster: Dull to vitreous; Crystal Structure: Foliated (gneissose banding); Cleavage: Poor, tends to break along bands; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. This specimen shows 'mylonitization' or shearing, and has been rounded by fluvial (river) or glacial transport.
Uses & applications
Used in construction as crushed stone or dimension stone for countertops and flooring. River pebbles like this are used in landscaping and aquariums.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The banding occurs because minerals separate under intense heat and pressure.
Field identification & locations
Identify in the field by distinct 'salt and pepper' banding (foliation) where dark minerals like biotite or hornblende align in layers against lighter quartz or feldspar. Commonly found in mountainous regions or glacial till.
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