metamorphic
Gneiss
Metamorphic rock of high-grade regional metamorphism
Hardness: 6-7 on Mohs scale; Color: typically banded or streaked with alternating light (quartz, feldspar) and dark (biotite, hornblende) minerals; Luster: dull to vitreous; Crystal Structure: foliated; Cleavage: poor, breaks along irregular surfaces; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded or streaked with alternating light (quartz, feldspar) and dark (biotite, hornblende) minerals; Luster: dull to vitreous; Crystal Structure: foliated; Cleavage: poor, breaks along irregular surfaces; Specific Gravity: 2.6-2.9
Formation & geological history
Formed by high-pressure, high-temperature regional metamorphism of pre-existing igneous or sedimentary rocks. This specimen shows characteristic gneissic banding and iron staining on the surface, typical of continental crust over billions of years of geological history.
Uses & applications
Commonly used as crushed stone in construction, road aggregates, and occasionally as decorative dimension stone for countertops or gravestones if the banding is particularly aesthetic.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada being dated to approximately 4.03 billion years old. The term 'gneiss' comes from an Old High German verb meaning 'to spark' or 'to glow'.
Field identification & locations
Identified in the field by its distinct mineral banding (foliation) where minerals are separated into dark and light layers. Found globally in the roots of mountain ranges and ancient continental shields. Look for visible crystalline textures and lack of easy splitting like schist or slate.
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