metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and black; Lustre: dull to vitreous; Structure: foliated with distinct banding (gneissose banding); Cleavage: poor; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- typically banded gray, white, and black
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded gray, white, and black; Lustre: dull to vitreous; Structure: foliated with distinct banding (gneissose banding); Cleavage: poor; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale). It is subjected to high temperatures and pressures, typically associated with mountain-building events.
Uses & applications
Used primarily as a crushed stone in construction for road bases and concrete. Select high-quality varieties are used as dimension stone for counter tops, flooring, and building facades.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dated to approximately 4.03 billion years old. The term comes from the German word 'Gneis' meaning 'to spark'.
Field identification & locations
Identify by looking for distinct 'zebra' stripes or banding where light (quartz/feldspar) and dark (biotite/hornblende) minerals separate. It is commonly found in the cores of mountain ranges.
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