metamorphic
Garnet Gneiss
Garnetiferous quartzo-feldspathic gneiss
Hardness: 6-7.5 (Mohs); Color: Tan to light gray matrix with dark red/brown garnets and black biotite or amphibole streaks; Luster: Dull to vitreous; Crystal structure: Foliated; Cleavage: Poor/None for garnets
- Hardness
- 6-7
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7.5 (Mohs); Color: Tan to light gray matrix with dark red/brown garnets and black biotite or amphibole streaks; Luster: Dull to vitreous; Crystal structure: Foliated; Cleavage: Poor/None for garnets
Formation & geological history
Formed under high-pressure and high-temperature metamorphic conditions (regional metamorphism) altering sedimentary or igneous protoliths. Common in Precambrian shield areas.
Uses & applications
Informational specimen for teaching geology, architectural stone (when in large slabs), or minor gemstone interest if garnets are high quality.
Geological facts
The red spots are Almandine garnets, which are often used as abrasives in sandpaper and waterjet cutting due to their hardness.
Field identification & locations
Identify by looking for 'foliation' (banding) and distinctive round, dark-red mineral inclusions embedded in a hard, crystalline matrix. Found in glacial till and mountain ranges.
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