metamorphic
Gneiss (specifically a Banded Gneiss pebble)
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Alternating bands of light minerals (quartz, feldspar) and dark minerals (biotite, hornblende); Luster: Sub-vitreous to dull; Crystal structure: Foliated (gneissose banding); Cleavage: Poor, though it may split along dark micaceous layers; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating bands of light minerals (quartz, feldspar) and dark minerals (biotite, hornblende); Luster: Sub-vitreous to dull; Crystal structure: Foliated (gneissose banding); Cleavage: Poor, though it may split along dark micaceous layers; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/sandstone) protoliths. It undergoes intense heat and pressure deep within the Earth's crust, often associated with mountain-building events (orogenies). This specimen appears water-worn, likely tumbled in a river or coastal environment after eroding from bedrock.
Uses & applications
Used primarily in the construction industry as crushed stone for road base or as dimension stone for counter tops and building facades. Tumbled pebbles like this are frequently used in landscaping or as decorative stones in aquariums and garden paths.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. The term 'gneiss' comes from an Old High German verb meaning 'to spark' or 'to glisten' because of its mineral content.
Field identification & locations
Identify in the field by looking for distinct 'gneissose banding'—compositional layers of different colors that do not easily split (unlike schist). This specimen is a common discovery in glacial till or riverbeds in mountainous regions like the Appalachians, Rockies, or Scandinavian Shield.
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