metamorphic
Gneiss (specifically a weathered or stream-worn specimen)
Gneiss (Foliated Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Gray, tan, and dark brown ribbons; Luster: Dull to earthy (weathered surface); Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: None, but breaks along foliation planes.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, tan, and dark brown ribbons
- Luster
- Dull to earthy (weathered surface)
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, tan, and dark brown ribbons; Luster: Dull to earthy (weathered surface); Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: None, but breaks along foliation planes.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under intense heat (over 600°C) and pressure. This specific specimen shows signs of physical weathering, likely from fluvial (river) transport, during the Holocene or Pleistocene epochs.
Uses & applications
Used primarily as crushed stone for road construction, concrete aggregate, and as 'dimension stone' for flooring, gravestones, and landscape features. High-quality pieces are used for kitchen countertops.
Geological facts
Gneiss is some of the oldest rock on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. The banding occurs because different minerals migrate and segregate into layers during extreme pressure.
Field identification & locations
Identify in the field by looking for 'zebra stripes' or distinct light (quartz/feldspar) and dark (biotite/amphibole) mineral bands. Commonly found in the roots of eroded mountain ranges or glacial till. This specimen's rounded edges suggest it was tumbled in a stream or riverbed.
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