metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating light and dark layers (foliation); Luster: dull to vitreous; Crystal structure: coarse-grained, granular minerals; Cleavage: poor; 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 with alternating light and dark layers (foliation); Luster: dull to vitreous; Crystal structure: coarse-grained, granular minerals; Cleavage: poor; Specific gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like mudstone/shale). Formed under extreme heat and pressure conditions, often during mountain-building events (orogenies).
Uses & applications
Used extensively in construction as dimension stone, crushed stone for road bases, and for architectural facing, countertops, and flooring due to its durability and aesthetic banding.
Geological facts
Gneiss is one of the oldest rocks found on Earth, with some specimens in the Acasta Gneiss complex dated to over 4 billion years old. The term comes from a Middle High German verb meaning 'to spark' or 'to glisten'.
Field identification & locations
Identified by 'gneissic banding' – distinct light-colored layers (quartz/feldspar) and dark-colored layers (biotite/amphibole). Commonly found in the crystalline cores of many mountain ranges and Precambrian shield areas. Collectors look for well-defined, wavy bands or garnet inclusions.
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