metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 Mohs; Color: Banded pink (feldspar), white (quartz), and black/grey (biotite/hornblende); Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 Mohs
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Banded pink (feldspar), white (quartz), and black/grey (biotite/hornblende); Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissic banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by the high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) under extreme heat and pressure. It often dates back to the Precambrian era depending on the shield region.
Uses & applications
Used in construction as crushed stone for road bases, as dimension stone for countertops, flooring, and gravestones, and as decorative landscape rock.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada estimated to be over 4 billion years old. The term comes from the Middle High German verb 'gneist' meaning 'to spark'.
Field identification & locations
Identified by 'gneissic banding' where minerals are segregated into distinct light and dark layers. Common in continental shields and mountain roots. For collectors, look for contrasting mineral layers and lack of schistose mica-sheen.
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