metamorphic
Banded Gneiss or Migmatite Pebble
Gneiss / Migmatite
AI-generated identification · may be incorrectHardness varies (generally 6-7), distinct banding with light (felsic) and dark (mafic) layers. The light layer appears to be quartz and feldspar, while the dark layer is likely rich in biotite or hornblende. Luster is vitreous to dull depending on weathering.
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Physical properties
Hardness varies (generally 6-7), distinct banding with light (felsic) and dark (mafic) layers. The light layer appears to be quartz and feldspar, while the dark layer is likely rich in biotite or hornblende. Luster is vitreous to dull depending on weathering.
Formation & geological history
Formed through high-grade regional metamorphism, where pre-existing rocks were subjected to intense heat and pressure deep within the Earth's crust, causing minerals to segregate into bands. Later weathered and rounded by water action, likely in a river or beach environment.
Uses & applications
Commonly collected by rockhounds for its aesthetic banding. Large deposits are used as crushed stone for construction or dimension stone.
Geological facts
The distinct separation of light and dark minerals is called gneissic banding. If the rock partially melted during formation, it is called a migmatite.
Field identification & locations
Identify by the alternating light and dark bands. Commonly found in areas with ancient exposed continental crust or as glacial erratics and river rocks in outwash plains.
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