metamorphic

Gneiss (specifically a weathered or stream-worn specimen)

Gneiss (Foliated Metamorphic Rock)

AI-generated identification · may be incorrect

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)

Identified March 24, 2026More metamorphic →

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Physical properties

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.