metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 (Mohs scale); Color: Typically alternating light and dark bands (foliated); Luster: Generally dull but can show vitreous sparkle from mica or quartz; Crystal Structure: Coarse-grained, crystalline; Cleavage: Poor, but splits along bands; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Typically alternating light and dark bands (foliated)
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Typically alternating light and dark bands (foliated); Luster: Generally dull but can show vitreous sparkle from mica or quartz; Crystal Structure: Coarse-grained, crystalline; Cleavage: Poor, but splits along bands; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-pressure and high-temperature regional metamorphism of igneous or sedimentary rocks (protoliths like granite or shale). These specimens are often billions of years old, part of the Earth's crystalline basement.
Uses & applications
Primarily used as a crushed stone in construction for road bases and concrete. Higher-quality slabs are used for architectural facings, flooring, and kitchen countertops (often sold commercially as granite).
Geological facts
The Acasta Gneiss in Canada is considered one of the oldest known intact crustal rocks on Earth, dating back approximately 4.03 billion years.
Field identification & locations
Identify in the field by looking for 'gneissose banding' (distinct layers of minerals). It is common in mountainous regions like the Appalachians, Himalayas, and Alps. Collectors look for unique banding patterns or included crystals like garnets.
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