metamorphic
Weathered gneiss (likely)
Felsic banded gneiss, composed mainly of quartz and feldspar with mica
AI-generated identification · may be incorrectCoarse-grained, gray-tan rock with faint mineral banding and dark streaks; minerals are typically Mohs 6–7 (quartz and feldspar), with vitreous quartz and dull-weathered surfaces. The photograph does not resolve individual crystals, so identification is tentative.
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Physical properties
Coarse-grained, gray-tan rock with faint mineral banding and dark streaks; minerals are typically Mohs 6–7 (quartz and feldspar), with vitreous quartz and dull-weathered surfaces. The photograph does not resolve individual crystals, so identification is tentative.
Formation & geological history
Forms when pre-existing rock undergoes high-grade heat and pressure, causing minerals to segregate into bands; gneisses range from ancient Precambrian to younger ages. The exposed boulder is weathered in place or may have been transported by ice.
Uses & applications
Gneiss is used as dimension stone, paving, aggregate, and landscape stone; this unworked specimen has little commercial value.
Geological facts
Gneissic banding reflects mineral separation during metamorphism and can resemble layering in sedimentary rocks, but is typically coarser and more irregular.
Field identification & locations
Look for coarse, interlocking light and dark mineral bands; confirm with a fresh surface and hand lens, since weathered granite or other metamorphic rocks can look similar. Gneiss occurs worldwide in ancient continental crust and glacial deposits.
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