metamorphic
Gneissic boulder
Gneiss, typically quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray, coarse-grained, with dark and light compositional banding, foliated texture, and vitreous to dull luster. Quartz and feldspar provide interlocking crystals; mica may form glittering cleavage surfaces.
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Physical properties
Hardness about 6–7 Mohs; gray, coarse-grained, with dark and light compositional banding, foliated texture, and vitreous to dull luster. Quartz and feldspar provide interlocking crystals; mica may form glittering cleavage surfaces.
Formation & geological history
Formed when preexisting granite or sedimentary rock underwent high-grade metamorphism involving intense heat and pressure, commonly during Precambrian to Paleozoic mountain-building. The exposed boulder may have been transported or shaped by glacial erosion.
Uses & applications
Used locally as dimension stone, landscaping rock, building aggregate, and occasionally polished architectural stone; attractive banded pieces are collected but rarely used as gemstones.
Geological facts
Gneiss is identified by mineral segregation into alternating bands rather than the thin layering typical of schist. Its strong foliation records deep crustal deformation.
Field identification & locations
Look for coarse, alternating light quartz-feldspar and dark mica/amphibole bands, a massive weather-resistant surface, and no vesicles. Common in ancient continental shields and glaciated regions such as New England, Canada, Scandinavia, and the Adirondacks.
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