metamorphic
Garnet Gneiss
Garnetiferous Gneiss
Hardness: 6.5-7.5 (garnets) / 6-7 (overall rock); Color: Grey to dark grey matrix with red to brownish-red garnet inclusions; Luster: Vitreous to dull; Crystal structure: Gneissose foliation with porphyroblastic garnets; Specific gravity: 2.7–3.2
- Hardness
- 6
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6.5-7.5 (garnets) / 6-7 (overall rock); Color: Grey to dark grey matrix with red to brownish-red garnet inclusions; Luster: Vitreous to dull; Crystal structure: Gneissose foliation with porphyroblastic garnets; Specific gravity: 2.7–3.2
Formation & geological history
Formed under high-pressure and high-temperature metamorphic conditions (regional metamorphism) from a protolith of shale or igneous rock. These rocks are typically deep tectonic remnants of ancient mountain building events.
Uses & applications
Used as decorative building stone, high-quality abrasives (garnet content), and sometimes crushed stone for construction. Larger garnets may be collected as gemstone specimens.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens dating back over 4 billion years. The presence of garnets indicates that the rock was subjected to intense heat and pressure during its formation.
Field identification & locations
Identified in the field by its distinct banded 'gneissose' texture and the presence of small, round, red-colored garnet crystals embedded in a crystalline matrix. Commonly found in metamorphic belts like the Appalachians, Alps, and Himalayas.
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