metamorphic
Gneiss (Pebble)
Gneiss
Hardness: 6-7 on Mohs scale. Color: Alternating bands of light and dark minerals (salt and pepper appearance). Luster: Dull to vitreous. Crystal structure: Foliated with distinct banding. 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: Alternating bands of light and dark minerals (salt and pepper appearance). Luster: Dull to vitreous. Crystal structure: Foliated with distinct banding. Specific Gravity: 2.6–2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. Exposed to intense heat and pressure, causing minerals to recrystallize and segregate into bands. These specimens are often Precambrian to Paleozoic in age.
Uses & applications
Commonly used as crushed stone in construction, road base, and landscaping. High-quality slabs are used for countertops and ornamental building stone.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years old. The banding is evidence of the extreme geologic forces the rock survived.
Field identification & locations
Identify in the field by looking for distinct 'gneissose banding' (layers of different colors). It won't split along planes as easily as schist. Commonly found in shield areas of continents or mountainous regions.
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