metamorphic
Gneiss (Gneissic Granite Pebble)
Metamorphic rock rich in Quartz, Feldspar, and Mica
Hardness: 6-7 on Mohs scale. Color: Gray, tan, white, and dark speckles in subtle foliation bands. Luster: Dull to vitreous on fresh surfaces. Texture: Medium to coarse-grained, display characteristic metamorphic banding (gneissic foliation). Specific Gravity: 2.6 - 2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous on fresh surfaces
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Gray, tan, white, and dark speckles in subtle foliation bands. Luster: Dull to vitreous on fresh surfaces. Texture: Medium to coarse-grained, display characteristic metamorphic banding (gneissic foliation). Specific Gravity: 2.6 - 2.9.
Formation & geological history
Formed through high-grade regional metamorphism under extreme heat and pressure from pre-existing sedimentary or igneous rocks (such as granite or sandstone). The stream-rounded shape indicates subsequent erosion and transport by water or glacial activity.
Uses & applications
Crushed stone for road construction, building foundations, landscaping river rocks, and decorative garden gravel.
Geological facts
Gneiss is one of the oldest rock types found on Earth. Some Acasta Gneiss specimens from Canada have been dated to over 4 billion years old, making them among the oldest intact crustal material known.
Field identification & locations
Identify in the field by looking for alternating light-colored bands (quartz and feldspar) and dark-colored bands (biotite and hornblende) along with a smoothed, water-worn exterior if found in riverbeds. Very common in glaciated regions and mountainous river drainages.
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