metamorphic
Feldspathic Gneiss
Feldspathic Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 (Mohs scale); Color: Banded pink/orange and dark grey/black; Luster: Dull to vitreous; Structure: Foliated with distinct mineral banding; Cleavage: Generally none, breaks along foliation planes. Specific gravity: 2.6-2.9.
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded pink/orange and dark grey/black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded pink/orange and dark grey/black; Luster: Dull to vitreous; Structure: Foliated with distinct mineral banding; Cleavage: Generally none, breaks along foliation planes. Specific gravity: 2.6-2.9.
Formation & geological history
Formed from the regional metamorphism of igneous or sedimentary protoliths under high temperature and pressure. The pink bands are likely potassium feldspar (orthoclase), while dark bands are biotite or hornblende. These specimens are often billions of years old, dating from the Precambrian era.
Uses & applications
Used in construction as crushed stone or dimension stone (granite-like), high-end landscaping, and as decorative river stones or palm stones for collectors.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. The term 'gneiss' comes from an Old High German verb meaning 'to spark' or 'bright' because of its lustrous mineral content.
Field identification & locations
Identify by looking for 'gneissose banding' (alternating light and dark layers). Found worldwide in the hearts of mountain ranges and ancient continental shields. Collectors look for high contrast and smooth rounding by water erosion.
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