metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and pinkish/tan; Luster: Dull to vitreous; Crystal Structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, white, and pinkish/tan
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, white, and pinkish/tan; Luster: Dull to vitreous; Crystal Structure: Foliated with distinct banding (gneissose banding); Cleavage: Poor; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale) subjected to intense heat and pressure deep within the Earth's crust. Most specimens range from hundreds of millions to billions of years old.
Uses & applications
Commonly used as crushed stone in road construction and landscaping, or as dimension stone for building facades, flooring, and cemetery markers. Polished slabs are occasionally used for countertops.
Geological facts
Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada dated to about 4.03 billion years old. The term comes from an Old High German word meaning 'to spark' or 'to glisten'.
Field identification & locations
Identified by 'gneissose banding' where minerals are segregated into dark and light layers. Commonly found in the cores of mountain ranges and in glacial till in the northern United States and Europe. Collectors look for sharp contrast in the bands and presence of garnet crystals.
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