metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and black with occasional pink or tan; Luster: Dull to vitreous; Crystal Structure: Foliated (gneissic banding); Cleavage: Poorly developed along bands.
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded grey, white, and black with occasional pink or tan
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and black with occasional pink or tan; Luster: Dull to vitreous; Crystal Structure: Foliated (gneissic banding); Cleavage: Poorly developed along bands.
Formation & geological history
Formed through high-pressure, high-temperature metamorphism of granite or sedimentary rocks like shale. It typically forms deep within the earth's crust during mountain-building events (orogeny).
Uses & applications
Used primarily as decorative construction stone, crushed stone for road bases, flooring, and kitchen countertops due to its durability and aesthetic banding.
Geological facts
Gneiss is one of the oldest rock types found on Earth. Some of the oldest known continental crust, the Acasta Gneiss in Canada, is approximately 4 billion years old.
Field identification & locations
Identify in the field by looking for distinct mineral segregation into light and dark bands (foliation). It is extremely common in continental shields and ancient mountain ranges. Durable and heavy specimen.
