metamorphic
Gneiss (specifically a Hornblende Gneiss or Micaceous Gneiss)
Gneiss (metamorphic rock composed primarily of quartz, feldspar, and mafic minerals)
Hardness: 6-7 on Mohs scale; Color: Banded grey, black, and white; Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissic banding); Cleavage: Poor, typically breaks irregular or along planes; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Banded grey, black, and white
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded grey, black, and white; Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissic banding); Cleavage: Poor, typically breaks irregular or along planes; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme pressure and temperature. This process segregates minerals into light and dark layers. Often of Precambrian or Paleozoic age.
Uses & applications
Used in construction as crushed stone for road base, building stone for cladding or countertops, and as ornamental landscaping stone.
Geological facts
Gneiss is one of the oldest known rocks on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding occurs because minerals recrystallize and migrate into layers during metamorphism.
Field identification & locations
Identify in the field by looking for 'salt and pepper' banding or stripes that are thicker than schist. It is commonly found in the cores of mountain ranges and ancient continental shields. Collectors look for sharp, high-contrast banding.
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