metamorphic
Garnet Gneiss
Gneiss containing Almandine Garnet
Hardness: 6-7.5 (Mohs scale); Color: light gray or white matrix with dark banding and reddish-brown garnets; Luster: dull to vitreous; Structure: foliated with distinct banding.
- Hardness
- 6-7
- Luster
- dull to vitreous
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Physical properties
Hardness: 6-7.5 (Mohs scale); Color: light gray or white matrix with dark banding and reddish-brown garnets; Luster: dull to vitreous; Structure: foliated with distinct banding.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing sedimentary or igneous rocks under intense heat and pressure, often during mountain-building events.
Uses & applications
Used as a decorative stone in landscaping, dimension stone for architecture, and occasionally as an abrasive if garnet content is high.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens dating back over 4 billion years. The dark spots visible are garnet crystals, which are index minerals used to determine metamorphic grade.
Field identification & locations
Identify by looking for distinct 'gneissic banding' and small reddish crystals. It is commonly found in the roots of mountain ranges (e.g., Appalachians, Alps, Himalayas) and crystalline shields.
