metamorphic
Gneiss
Foliated Metamorphic Gneiss
Hardness: 6-7 (Mohs). Color: Alternating bands of grey, tan, and dark minerals. Luster: Dull to pearly. Structure: Foliated with distinct banding (gneissic banding). Cleavage: Poor, usually breaks along uneven surfaces.
- Hardness
- 6-7 (Mohs)
- Color
- Alternating bands of grey, tan, and dark minerals
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 (Mohs). Color: Alternating bands of grey, tan, and dark minerals. Luster: Dull to pearly. Structure: Foliated with distinct banding (gneissic banding). Cleavage: Poor, usually breaks along uneven surfaces.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. This specimen shows significant heat and pressure deformation, likely dating back hundreds of millions of years from deep within the Earth's crust.
Uses & applications
Used primarily as crushed stone for road construction and as decorative building stone or facing. High-quality specimens are sometimes used for kitchen countertops or floor tiles.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens in Canada dated to over 4 billion years old. The banding is caused by the segregation of light and dark minerals during intense pressure.
Field identification & locations
Identify by looking for distinct, thick bands of different colored minerals that are not easily crumbled. Commonly found in the cores of mountain ranges or in glacial deposits. This rounded specimen suggests it was eroded by water in a river or shore.
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