metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded colors of grey, white, and pink; Luster: dull to vitreous; Crystal Structure: foliated; Cleavage: poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- typically banded colors of grey, white, and pink
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded colors of grey, white, and pink; Luster: dull to vitreous; Crystal Structure: foliated; Cleavage: poor; Specific Gravity: 2.6-2.9
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale). Subjected to intense heat and pressure, causing minerals to recrystallize and segregate into bands.
Uses & applications
Used primarily as decorative stone for countertops, flooring, and gravestones. Also used as crushed stone in road construction and landscaping.
Geological facts
The Acasta Gneiss in Canada is one of the oldest known crustal rock formations in the world, dated at approximately 4.03 billion years old.
Field identification & locations
Identified in the field by its distinct alternating dark (biotite, amphibole) and light (quartz, feldspar) bands, known as gneissic banding. Common in mountainous regions or ancient continental shields.
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