metamorphic
Banded Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 Mohs; Color: Dark grey to black with tan or cream bands; Luster: Dull to sub-vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 Mohs
- Color
- Dark grey to black with tan or cream bands
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to black with tan or cream bands; Luster: Dull to sub-vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through the high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure. Most gneiss specimens are Proterozoic or Archean in age, dating back hundreds of millions to billions of years.
Uses & applications
Used as a decorative stone in landscaping, crushed stone for construction, countertops, and occasionally as a polished 'worry stone' or collector specimen.
Geological facts
Gneiss is one of the oldest rocks found on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The term 'gneiss' comes from an old German word meaning 'sparkling' or 'to glisten'.
Field identification & locations
Identify by looking for 'gneissic banding' where minerals are segregated into distinct light and dark layers. Commonly found in the roots of mountain ranges and glacial till across North America and Europe.
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