metamorphic
Gneiss (with Garnet)
Gneiss (Feldspar-Quartz-Biotite-Garnet Metamorphic Rock)
Hardness 6-7 on Mohs scale. Color ranges from dark grey, brown, to dark blue-black with lighter flecks of quartz/feldspar and reddish garnet inclusions. Luster is dull to slightly vitrous on polished or wet surfaces. Foliated texture with medium-to-coarse grain size.
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Physical properties
Hardness 6-7 on Mohs scale. Color ranges from dark grey, brown, to dark blue-black with lighter flecks of quartz/feldspar and reddish garnet inclusions. Luster is dull to slightly vitrous on polished or wet surfaces. Foliated texture with medium-to-coarse grain size.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing sedimentary or igneous rocks (such as granite or shale) subjected to intense heat and pressure deep within Earth's crust.
Uses & applications
Used as a dimension stone, crushed stone for road construction, landscaping, building facades, and as a decorative stone specimen for mineral collectors.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is over 4 billion years old. Its distinct alternating dark and light mineral bands are called gneissic banding.
Field identification & locations
Look for distinct light and dark mineral alignment or banding, along with small red or brownish garnet grains embedded within a hard crystalline matrix. Found abundantly in ancient shield regions and mountain belts globally.
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