metamorphic
Granite Gneiss with Garnet
Garnetiferous Gneiss
Hardness: 6-7.5 (Mohs scale); Color: Mottled white, grey, tan, and reddish-pink patches; Luster: Vitreous to dull; Crystal structure: Foliated or granoblastic; Specific gravity: 2.6-3.2
- Hardness
- 6-7
- Color
- Mottled white, grey, tan, and reddish-pink patches
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7.5 (Mohs scale); Color: Mottled white, grey, tan, and reddish-pink patches; Luster: Vitreous to dull; Crystal structure: Foliated or granoblastic; Specific gravity: 2.6-3.2
Formation & geological history
Formed through the high-grade regional metamorphism of igneous rocks like granite or sedimentary rocks like shale. The red spots are likely almandine garnets formed under high pressure and temperature in the Earth's crust.
Uses & applications
Used in construction (crushed stone), decorative landscaping, and occasionally as minor gemstones if the garnets are large and clear. Primarily a collector's specimen when found in river-tumbled form.
Geological facts
Gneiss is one of the oldest rock types on Earth, with some specimens in Northern Canada dated to over 4 billion years old. The 'eyes' or spots of garnet are indicator minerals for the intensity of metamorphism.
Field identification & locations
Identify in the field by the salt-and-pepper appearance combined with dark red crystalline inclusions (garnets) and a smooth, water-worn texture if found near coastline or rivers. Common in high-grade metamorphic terrains like the Appalachians or Scandinavia.
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