metamorphic
Quartz-rich gneiss
Banded quartz–feldspar–mica gneiss; commonly containing biotite and muscovite
AI-generated identification · may be incorrectHardness about 6–7 Mohs; coarse-grained, gray, cream, and rusty-brown, with glassy quartz and mica sparkle. Strong compositional banding and foliation are visible; cleavage is mainly in mica, while quartz has none.
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Physical properties
Hardness about 6–7 Mohs; coarse-grained, gray, cream, and rusty-brown, with glassy quartz and mica sparkle. Strong compositional banding and foliation are visible; cleavage is mainly in mica, while quartz has none.
Formation & geological history
Formed when preexisting granite or sedimentary rock underwent high-grade metamorphism, recrystallizing under intense heat and pressure. Its exact age cannot be determined from the photograph; many such rocks are Precambrian to Paleozoic.
Uses & applications
Used as crushed aggregate, building stone, landscaping rock, and occasionally as polished decorative stone. Attractive, well-banded pieces have modest collector value.
Geological facts
The light bands are quartz and feldspar, whereas darker bands are mica- or amphibole-rich; rusty patches commonly reflect weathering of iron-bearing minerals.
Field identification & locations
Identify it by alternating light and dark mineral bands, visible interlocking grains, and mica flakes—unlike layered sedimentary rock, the bands are metamorphic foliation. Common in ancient continental shields and mountain belts; typical value is $5–$30 for a hand specimen, or $30–$150 for an attractive polished piece.
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